Below is a comprehensive elaboration of Acts 2:14–21 for your third discussion. I’ve organized it so you can work through it week by week or in one sitting, and I’ve included a set of discussion questions and an LCMS-specific confessional perspective at the end.
Acts is the second volume of Luke’s two-part work. In Luke, the Gospel, Jesus teaches and establishes the Church by death, resurrection, and ascension. In Acts, the Church is sent by the risen and ascended Christ with the power of the Holy Spirit to bear witness “in Jerusalem, and in all Judea and Samaria, and to the end of the earth” (1:8).
By the time we arrive at Acts 2:
That question is answered in verses 14–21. Peter stands, addresses the crowd as “men of Judea and all who dwell in Jerusalem,” and gives the first formal proclamation of the resurrection — but notice that the first sermon actually opens with a prophecy, not with “Jesus is risen.” This is the key to the title of your study: the New is rooted in the Old. Peter does not begin with something new; he begins by locating the present moment within God’s ancient, already-promised story.
14 But Peter, standing up with the eleven, raised his voice and addressed them: “Men of Judea and all who dwell in Jerusalem, let this be known to you, and give heed to my words. 15 For these men are not drunk, as you suppose, since it is only the third hour of the day. 16 But this is what was spoken by the prophet Joel: 17 ‘And in the last days it shall be, God declares, that I will pour out my Spirit on all flesh, and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams; 18 yes, and on my male servants and female servants in those days I will pour out my Spirit, and they shall prophesy. 19 And I will show wonders in the heavens above and signs on the earth below, blood, and smoke, and fire; 20 the sun shall be turned into darkness, and the moon into blood, before the day of the Lord comes, the great and magnificent day. 21 And it shall be that everyone who calls on the name of the Lord shall be saved.’ 22 For Moses, as I have told you, said, ’The Lord God will raise up for you a prophet like me from among your brothers. Him you shall listen to in all that he may say to you.”
Peter’s quotation of Joel 2:28–32 (in the Septuagint, Joel 3:1–5) is the heart of the passage. Almost every interpretive issue in these eight verses turns on what Joel wrote, why Peter is quoting it now, and what “last days” and “the day of the Lord” mean in this setting.
Peter’s formula — stasas de ho Petros kai ho hendiakos … ephezeto epeideixai — is the formal opening of a public proclamation (cf. Lev 10:1; 1 Sam 2:15; 2 Sam 17:16; Acts 17:11, 22:10, 25:10). It is the same kind of solemn address used by prophets (cf. Jer 1:2, “Hear the word of the Lord”) and by Jesus himself. Two things deserve emphasis:
The third hour of the day (roughly 9 a.m.) is the time of the morning temple sacrifice (Ex 29:38–42; Num 28:1–6). To be publicly intoxicated at that hour in the temple precincts was not only undignified, it was a serious breach of cultic propriety (Isa 28:7–8; Sirach 31:30). Peter’s point is not primarily social etiquette; it is epistemological. If the disciples were drunk, what is happening is an accident. If they are sober, what is happening is an event with meaning. Peter’s first move is to force the crowd to take the event seriously enough to look for its meaning.
This is a useful homiletical principle worth teaching the group: the first task of apologetics is to establish that there is something to explain, and that the explanation is not a trivial one.
This is the hinge of the passage and the reason for your study’s title. Peter does not say, “This is something new; this is what God has decided to do.” He says, “this is what was spoken.” The event the crowd is witnessing — the wind, the flames, the foreign languages, the ecstatic speech — is the fulfillment of a specific, named, ancient prophecy. God’s present action is a continuation of his past word.
This is a very Jewish and very Lukan way of reading Scripture. Luke–Acts repeatedly presents the events of Jesus’s life, death, resurrection, and ascension as the fulfillment of the Law, Prophets, and Psalms (Lk 24:27, 44; Ac 3:18, 22; 13:29, 40; 26:22–23). The New is not a replacement; it is the root of the Old’s fruit. Every “this is” in Luke–Acts is a fulfillment citation.
For LCMS study, this is exactly the Reformation principle of Sola Scriptura operating in practice: the canon is one, continuous, and self-interpreting. The New Testament does not sit beside the Old as a rival canon; it sits within it as its appointed climax. Peter’s first instinct is not to invent a new category of “the Church” but to say, “the Church is the last days Joel was talking about.”
Peter quotes Joel 2:28–32 (LXX 3:1–5) almost verbatim, with small variations. Let’s work through it.
The Septuagint reads en tois eschatōs hēmerais — “in the last days.” Peter picks up exactly this phrase and applies it to the day of Pentecost. This is one of the most important christological statements in Acts: the “last days” have already begun. The eschatological “between the times” inaugurated by Christ’s resurrection is now present. Joel’s eschatological horizon is not merely future; it is breaking into the here and now.
This is a key Lutheran point: the Gospel is always present-tense. The “last days” are not a distant, theoretical era; they are the era in which the Word of the Cross is being preached. Every altar call, every Sunday sermon, every baptism, every Lord’s Supper is within the “last days.” The Kingdom has both “already” and “not yet” dimensions, and Pentecost is the hinge.
This is the most radical expansion in the passage. Joel’s original context (2:2–17) is one of judgment — the “day of the Lord” arriving like a consuming locust plague. The remedy is corporate repentance (2:12–17), and the reward for that repentance is the Spirit poured out as restoration. Joel’s original audience is a specific, post-exilic, covenantal Israel. The phrase kol sār (“all flesh”) in the Hebrew text is a standard covenant formula, but in its fuller theological import it points beyond the boundaries of Israel.
Peter, quoting this on Pentecost, makes the point explicit: the Spirit is now poured out not only on a priestly or prophetic elite, but on sons and daughters, young men and old men, male and female servants. This is a deliberately socially flattening list. It includes people who, in the first-century Jewish culture Peter’s audience inhabited, had very little access to the prophetic vocation:
Theologically, this is a great equalizer: the Spirit is not a reward for religious status, ethnic privilege, or social position. It is a free gift, poured out on “all flesh” (pasa sarx in the LXX and Greek). In an LCMS frame, this is the Gospel at its purest: justification by grace alone through faith alone, apart from works of the Law. The Spirit is not given to the righteous; it is given so that the recipient can believe and be righteous.
This also sets the stage for the radical inclusivity Acts will demonstrate in subsequent chapters: Peter and Cornelius (10), the Samaritans (8), the Ethiopian eunuch (8), Paul and the Gentiles (13–28). Pentecost is the door opening, not the door closing.
These three clauses are drawn from a well-known prophetic triad in the Hebrew Bible:
These are the three classical modes of Old Testament prophetic encounter. Joel promises that in the last days, all three modes will be democratized — they will no longer be restricted to a professional class. The effect in Acts 2 is that the disciples’ ecstatic speech in “other languages” (2:4) is the visible sign that this democratization is happening. The Spirit is so present that even people with no training in the modes of revelation are now participating in them.
Peter’s audience — Diaspora Jews — would have known this list as a covenantal hope, not a present reality. Peter says, it is happening now.
The LXX and Acts both include the servant clause. The Hebrew of Joel actually has ’ebadî wəʿamātay (“my male servants and my maidservants”), which is a natural covenant inclusion. Peter’s inclusion of this in his Pentecost sermon is a bold claim: the Spirit is not limited to the free, the educated, the ethnic Israelites. The Gospel, as Acts will demonstrate, crosses every boundary that first-century Judaism drew around religious life.
In the cultural context of Peter’s audience — a world where slavery was a legal and economic institution (cf. Roman domus servorum; cf. Eph 6:5; Col 4:1) — this is a radical statement. The Spirit’s outpouring does not depend on social station.
These are cosmic disturbances associated with the “day of the Lord” throughout the prophetic literature:
Peter’s quotation of Joel 2:30–31 (LXX 3:3–4) is drawing on this whole cluster of “day of the Lord” imagery. The key point is that the “day of the Lord” is not merely a future event. In the prophetic tradition, it is a recurring theme: it has already happened at the exile (Lam 1:15; 2:22), it is happening in the prophetic present (Isa 10:3–11; Jer 46:10; Zeph 1:14–16), and it will happen in the eschaton. Peter is using the image of the “day of the Lord” to describe the present moment — the day of the resurrection and the day of Pentecost are within that framework.
The “wonders in the heavens” and “signs on the earth” are not necessarily a prediction of specific future apocalyptic events. In context, they are the visible signs that God’s judgment on sin and the salvation of the repentant are happening simultaneously. The wind, the flames, the foreign languages (2:2–4) are the “wonders in the heavens” and “signs on the earth” of Joel’s prophecy. Peter is saying: “You are witnessing the very signs Joel spoke of.”
This is a critical point for the study group: Pentecost is not merely a “blessing” event; it is also a judgment event. The Spirit is poured out, but the “day of the Lord” is also coming. The two are not opposed; they are two aspects of the same divine action. In an LCMS frame, this is the Law/Gospel dialectic in action: the Word convicts (Law) and saves (Gospel), and both are present at the same moment. The cross does both. The Spirit does both. Pentecost does both.
The phrase “the great and terrible day” (Hebrew yôm hāʾăḏōḏ hāgādôl; LXX hē hēmera tē megālē kai tē deina) is a formula that appears throughout the prophets (Zeph 1:14; Mal 3:10, 18, 23–24; Joel 2:11). Joel specifically uses it in 2:11: “Blow the trumpet in Zion; sanctify a fast day; call a solemn assembly.”
Peter’s application is that the “day of the Lord” has already come in the person of Jesus. Jesus’s death and resurrection are the day of the Lord. The “great and magnificent day” is not merely a future eschatological event; it is the day on which God has acted decisively in history to save his people. This is the Lutheran view of the already/not-yet: the day of the Lord is present in the preaching of the Cross, and it will be consummated at the resurrection of the dead.
The phrase “before the day of the Lord comes” (Joel 2:31; Ac 2:20) can be read in two ways, and both are theologically valid:
In an LCMS context, both readings are held in tension: the Gospel is a present reality (the “already”), but the eschatological consummation is still to come (the “not yet”). The tension is healthy and should not be forced into a single reading.
This is the climax of the quotation, and it is where the theological weight of the entire passage falls.
Peter is saying, in effect: the way of salvation is not by Jewish ritual purity, by ethnic identity, by social status, or by religious performance. It is by calling on the name of the Lord — i.e., by faith. This is the Reformation principle of Sola Fide (justification by faith alone) applied to the very first proclamation of the Gospel. The Gospel does not demand works; it promises salvation to those who believe.
The cultural context is critical: Peter’s audience is a crowd of Diaspora Jews — Jewish people from Rome, Alexandria, Cyrene, Egypt, Mesopotamia, Arabia, and other places (2:5–11). They are religious Jews who worship at the temple, keep the Law, and regard themselves as God’s chosen people. They are hearing from a Jewish rabbi (Peter) that:
This is a theological bomb in the first-century Jewish world. Peter is not saying “we Jews are still the chosen people”; he is saying “the chosen people are now everyone who believes in Jesus.” This is the seed of the inclusion of the Gentiles that will be fully developed in Acts 10–11 (Cornelius), Acts 15 (Jerusalem Council), and Acts 28 (Rome).
To understand why Peter’s quotation is so powerful, we need to understand what Joel was actually saying.
Joel is a short prophetic book (4 chapters) written in the period of the post-exilic or early Second Temple era. Its date is debated (some scholars place it in the 9th century BCE, others in the 5th century BCE), but its theological message is clear:
The key point is that Joel’s promise of the Spirit (2:18, 28–32) is conditioned on repentance. It is not an unconditional, free gift; it is God’s response to a covenant people who have turned back to him. This matters for our reading of Acts 2.
Peter does not emphasize the repentance condition in his quotation. He quotes the promise (2:17–21) but not the call to repentance (2:12–17). This is a deliberate rhetorical choice. Peter is not saying “the Spirit will be poured out if you repent.” He is saying “the Spirit has already been poured out, and you are witnessing the fulfillment of Joel’s promise.”
This is a crucial distinction, and it is one of the most important theological points in Acts. The promise is fulfilled first, and the call to repentance follows from the promise. In an LCMS frame, this is the Gospel-first order: the Word of grace precedes the call to conversion. The Law does not come first; the Gospel does. Peter’s sermon (2:22–36) will make this clear: he tells the crowd what God has done in Jesus, and then he calls them to repent because of what God has done, not as a prerequisite for what God will do.
This is the Reformation order: Gospel → Faith → Good Works, not Law → Obedience → Grace. Peter’s sermon follows this order precisely.
Peter quotes from the Septuagint (the Greek translation of the Hebrew Scriptures), which was the standard Scripture for Greek-speaking Jews in the Diaspora. Peter’s audience is precisely this audience: Greek-speaking, Diaspora Jews who worship at the temple but whose primary scriptural language is Greek. The fact that Peter quotes from the LXX is a pastoral choice: he is speaking their language, in both the literal and the spiritual sense.
The LXX of Joel 2:28–32 (Joel 3:1–5 in the LXX) is slightly different from the Hebrew in a few ways:
These small differences are not significant for our study, but they are worth noting: Peter is not reading the Hebrew text; he is reading the Greek text that his audience knows. This is an act of pastoral sensitivity and is a model for effective preaching.
Pentecost (Hebrew Shavuot, “weeks”; Greek Pentēkostē, “fiftieth day”) is one of the three major Jewish pilgrimage feasts (along with Passover and Tabernacles). It occurs 50 days after the first day of Unleavened Bread (Lev 23:15–16; Deut 16:9–10). Its significance is twofold:
The connection between Pentecost and the giving of the Torah is theologically rich. Peter’s sermon on the day of Pentecost is, in a sense, a new giving of the Word. At Sinai, God gave the Law to a specific people (Israel) in a specific place (the mountain). At Pentecost, God gives the Spirit to all people (the Church) in a specific place (Jerusalem). The Word is still central, but the Spirit is the agent of its application. The Law was given to condemn; the Spirit is given to empower. Both are rooted in God’s covenant faithfulness.
This is a powerful image for the study group: Pentecost is the “Sinai of the New Covenant.” The Law was the word of Sinai; the Gospel is the word of Pentecost. Both are the word of God. Both are necessary. The difference is that the Spirit makes the word of Pentecost life-giving in a way the Law of Sinai could not.
The crowd at Pentecost is not a random gathering. It is a pilgrimage crowd — Jews from across the known world who have come to Jerusalem for the feast (2:5–11). They include:
This is a cosmopolitan, multicultural, multilingual crowd. They are not a homogeneous group; they represent the full spread of the Jewish Diaspora. The fact that Peter’s audience is this diverse is theologically significant: the Gospel is not for a single ethnic group; it is for the whole world.
The significance of the “other languages” (2:4, 6, 8) in this context is that the crowd is hearing the disciples speak in their own native languages. This is not a miracle of translation; it is a miracle of communication. The Gospel is being communicated in a way that is accessible to every person in the crowd, regardless of their linguistic or cultural background. This is the first sign of the universal scope of the Gospel.
The “third hour of the day” (9 a.m.) is the time of the morning sacrifice in the temple (Ex 29:38–42; Num 28:1–6). This is a daily ritual that has been observed since the time of Moses. The fact that the Pentecost event takes place at this hour is not incidental. It is the moment when the temple is in full liturgical activity, when the priests are offering the morning sacrifice, and when the crowd is gathered for worship. The outpouring of the Spirit happens in the context of the temple liturgy. This is a deliberate theological statement: the Church is not a replacement for the temple; it is the fulfillment of the temple. The temple was a type; the Church is the antitype. The morning sacrifice was a shadow; the Spirit is the reality.
This is an important LCMS point: the Sacraments (Baptism and the Lord’s Supper) are not “inventions” of the Church; they are rooted in the covenantal practice of Israel. Baptism is the “circumcision” of the New Covenant (Col 2:11–12). The Lord’s Supper is the “Passover” of the New Covenant (1 Cor 5:7). The temple liturgy was the preparation; the Sacraments are the fulfillment.
The central theological claim of Acts 2:14–21 is that the New Testament is not a new story; it is the fulfillment of the Old. Peter’s first instinct is not to invent a new category of “the Church” or “the Spirit” or “the last days”; it is to say, “This is what Joel said would happen.” The New is rooted in the Old. This is the Reformation principle of Sola Scriptura in practice: the canon is one, continuous, and self-interpreting.
In an LCMS context, this means that preaching from the Old Testament is not merely historical or devotional; it is directly relevant to the Gospel. The Old Testament prophets are not “past” voices; they are present witnesses to the work of Christ. Every sermon on Acts 2:14–21 is, in effect, a sermon on Joel 2:28–32. The two texts are not separable.
Peter’s application of Joel’s “last days” to the present moment is one of the most important statements in Acts. The eschatological horizon is not merely future; it is breaking into the present. The “day of the Lord” is both a past event (the cross and resurrection), a present reality (the preaching of the Gospel and the outpouring of the Spirit), and a future consummation (the resurrection of the dead and the final judgment).
In an LCMS context, this is the doctrine of the already/not-yet. The Kingdom of God is both present and future. The believer lives in the tension between the two: they have been saved (past), they are being sanctified (present), and they will be glorified (future). The “last days” are the era in which all three of these are operative simultaneously.
The most radical claim of Acts 2:17–21 is that the Spirit is poured out on “all flesh.” This is not a limited, ethnic, or social category. It includes sons and daughters, young men and old men, male and female servants. In the first-century Jewish context, this is a theological revolution. The Gospel is not for Jews only; it is for everyone.
This is the seed of the mission to the Gentiles that will be fully developed in Acts 10 (Cornelius), Acts 15 (Jerusalem Council), and Acts 28 (Rome). The Church is not an ethnic group; it is a covenantal community defined by faith, not by blood. This is the Lutheran doctrine of the true Church (Ecclesia vera): the Church is the company of believers in the one true faith, the baptism, and the Lord’s Supper. It is not a political or ethnic entity; it is a spiritual community defined by the Word of God.
The Spirit’s outpouring in Acts 2 is the fulfillment of Joel’s promise and the inauguration of the New Covenant. The Spirit is not a “power” or a “force”; it is the third person of the Trinity, who proceeds from the Father and the Son (cf. John 14:16–17, 26; 15:26; 16:7–15). The Spirit is the agent of justification: it is the Spirit who creates faith, who applies the benefits of Christ’s work to the believer, and who sanctifies the believer for the sake of God’s glory.
In an LCMS context, the Spirit is the subject of the third article of the Apostles’ Creed: “I believe in the Holy Spirit, the holy Christian Church, the communion of saints, the forgiveness of sins, the resurrection of the body, and the life of the world to come.” The Spirit is the one who makes the Church, who forgives sins, who raises the dead, and who gives eternal life. The Spirit is not an abstraction; it is the active presence of God in the world.
Acts 2:14–21 is a perfect example of the Law/Gospel distinction in action. Peter’s sermon (2:22–36) moves from the Gospel (what God has done in Jesus) to the call to repentance (the Law) to the promise of salvation (the Gospel again). The Law and the Gospel are not opposed; they are dialectically related. The Law convicts; the Gospel saves. The Law kills; the Gospel gives life. Both are necessary. Both are present in the sermon.
In an LCMS context, this is the homiletical principle that governs all preaching: the Gospel must always be presented as a gift, not a demand. The Law is not the “bad news”; it is the “necessary diagnosis.” But the Gospel is the “cure.” Peter’s sermon follows this pattern: he tells the crowd what God has done (Gospel), then he calls them to repent (Law), then he tells them what God promises (Gospel). This is the Reformation order, and it is the order of the Church.
The climax of the passage — “everyone who calls on the name of the Lord shall be saved” (2:21) — is a clear statement of justification by faith alone. The promise of salvation is not conditioned on works, on ethnic identity, on social status, or on religious performance. It is conditioned on faith — i.e., on “calling on the name of the Lord.” This is the Reformation principle of Sola Fide applied to the very first proclamation of the Gospel.
In an LCMS context, this is the center of the Christian faith. The Augsburg Confession (Article IV) states: “We teach that man cannot be justified before God by his own merits, works, or satisfactions. But we are justified for Christ’s sake, who made satisfaction for us. Our faith grasps this grace and receives it.” Acts 2:21 is the New Testament foundation for this doctrine.
Peter’s sermon in Acts 2 is, in effect, a redefinition of the people of God. The people of God are no longer defined by ethnic identity (Jewish descent) or by religious performance (keeping the Law). They are defined by faith in Jesus Christ. The Church is the “true Israel” — the spiritual offspring of Abraham, the fulfillment of the covenant promises made to the patriarchs.
This is the Lutheran doctrine of the true Church (Ecclesia vera), which is defined not by human institutions but by the Word of God and the Sacraments. The Church is the community of those who hear the Word, who believe, who are baptized, and who partake of the Lord’s Supper. It is not a political or social entity; it is a spiritual community defined by the Holy Spirit.
Peter’s sermon is a Word-centered event. The Gospel is not communicated through ritual, through emotional experience, or through social action; it is communicated through the Word of God. Peter stands up and speaks. The Word is the primary means of grace. The Spirit works through the Word. This is the Lutheran principle of the Word as the means of grace (Gratiamedium), which is central to the LCMS confession and practice.
In an LCMS context, this means that preaching is not optional; it is essential. The Church exists to preach the Word. The Word exists to create faith. Faith exists to receive the Gospel. The entire structure of the Church is Word-centered.
Although Acts 2:14–21 does not explicitly mention the Sacraments, the theological context of the passage — the fulfillment of the covenant promises, the outpouring of the Spirit, the call to repentance and faith — is the sacramental context of the Church. The Sacraments (Baptism and the Lord’s Supper) are the visible signs of the invisible grace that God gives. They are the “words made visible.” In an LCMS context, this means that the Sacraments are not optional; they are essential. They are the means by which God gives and seals his grace to his people.
The universal scope of the Gospel in Acts 2:17–21 is the foundation of the mission of the Church. The Church exists not for itself, but to preach the Gospel to all people. This is the Great Commission (Matt 28:19–20) in action. The Church is a missionary community, called to carry the Gospel to the ends of the earth. In an LCMS context, this means that mission is not a program; it is the very nature of the Church.
Why does Peter open his sermon by quoting Joel instead of by announcing the resurrection directly? What does this tell us about the relationship between the Old and New Testaments?
What is the theological significance of the “last days” beginning at Pentecost? How does this change the way we understand the “already/not-yet” tension in Christian life?
Who is included in the list of those on whom the Spirit is poured out (sons and daughters, young men and old men, male and female servants)? What would this have meant to Peter’s first-century Jewish audience? What does it mean for us today?
How does Peter’s use of Joel’s “day of the Lord” imagery relate to the cross and resurrection of Jesus? In what sense is the “day of the Lord” both past, present, and future?
What is the theological significance of the phrase “everyone who calls on the name of the Lord shall be saved”? How does this relate to the Reformation doctrine of justification by faith alone?
How does the universal scope of the Gospel in Acts 2:17–21 challenge the idea that the Church is an ethnic or cultural community? How does this shape the mission of the Church today?
In what ways is Peter’s sermon a model for effective preaching? How does he move from the Word of God to the call to repentance to the promise of salvation? How does this compare to the way we preach and teach today?
How does the outpouring of the Spirit at Pentecost relate to the Sacraments of Baptism and the Lord’s Supper? In what sense are the Sacraments the “visible signs” of the invisible grace that God gives?
What does it mean that the Church is the “true Israel”? How does this shape our understanding of the relationship between Judaism and Christianity?
How does Acts 2:14–21 challenge us to think about the role of the Holy Spirit in the life of the Church? How is the Spirit’s work in us today a continuation of the work he began at Pentecost?
Preaching: Study Peter’s sermon as a model for effective, Word-centered preaching. How does he move from the Word of God to the call to repentance to the promise of salvation? How can we apply this pattern in our own teaching and preaching?
Mission: The universal scope of the Gospel in Acts 2:17–21 is a call to mission. How is our congregation carrying the Gospel to all people? What steps can we take to be more effective in our mission?
Sacraments: The outpouring of the Spirit at Pentecost is the foundation of the Sacraments. How do we value and participate in the Sacraments of Baptism and the Lord’s Supper? How can we better understand and appreciate their role in the life of the Church?
Community: The Church is not an ethnic or cultural community; it is a spiritual community defined by faith. How do we live out this reality in our congregation? How do we welcome and include people from all backgrounds into the family of God?
Prayer: The Spirit is the agent of faith and the source of grace. How do we pray for the work of the Spirit in our lives and in the world? How can we be more sensitive to the Spirit’s guidance in our daily lives?
The title of your study captures the central theological claim of Acts 2:14–21. The New Testament is not a new story; it is the fulfillment of the Old Testament. The Church is not a new institution; it is the true Israel, the fulfillment of the covenant promises made to Abraham, Isaac, and Jacob. The Spirit is not a new power; it is the agent of the covenant that God established with his people at Sinai and that he fulfilled in the person of Jesus Christ.
The “New” is rooted in the “Old.” The Gospel is rooted in the Law. The Church is rooted in Israel. The Sacraments are rooted in the covenant. The mission of the Church is rooted in the mission of the prophets. Every “new” thing in the Christian faith has its root in the ancient, covenantal, prophetic story of God’s people.
This is the Reformation principle of Sola Scriptura in action: the canon is one, continuous, and self-interpreting. The New Testament does not sit beside the Old as a rival canon; it sits within it as its appointed climax. Peter’s first instinct is not to invent a new category of “the Church” or “the Spirit” or “the last days”; it is to say, “This is what Joel said would happen.” The New is rooted in the Old.
That is the heart of the message of Acts 2:14–21, and it is the heart of the Christian faith.
May the Lord bless your study and may the Holy Spirit open the eyes of your heart to understand the hope to which he has called you (Eph 1:18).