Acts chapter 2 is well regarded as a foundation chapter in the New Testament. Among other things it touches on several facets of the work of God in saving man, including the work of Jesus Christ, the work of the Holy Spirit, and the work of man himself under God's plan... In some of these points we will also notice comments from Peter in his first epistle.

The Work of God (Acts 2)

1 The Work of Jesus Christ

• Christ's work on the cross
  In his sermon, Peter quoted Joel who prophesied the day in which "every one who calls on the name of the Lord shall be saved" (Acts 2:21, Joel 2:32). Peter then speaks of how Jesus was delivered up to be crucified according to God’s predetermined plan of salvation. Peter understood that "Christ died for our sins once for all, the just for the unjust" (1Pe 3:18).
• Christ’s work through the gospel
  Peter went on to declare the resurrection of Christ, and quoted David's prophecy in which the Messiah says to God, "You will not leave my soul in hades" (Acts 2:25-32). Christ is eternal. Even in Noah's time he "went and preached to the spirits in prison" (1Pe 3:19-20). Indeed, his Spirit moved all the prophets (1Pe 1:10-11). After his death he lived again to preach through Peter and the other apostles (Acts 2:33). He lives today and his gospel still saves today.
• Christ’s work in heaven
  Peter further declared the ascension of Christ to the throne of heaven, and again quoted David’s prophecy in which the Messiah says, "The Lord said to my Lord, 'sit at my right hand...'" (Acts 2:30,33-35, 1Pe 3:22). Clearly Christ is not sitting idle on the throne. Peter alludes to Christ's work in heaven when he says, "You are a royal priesthood" (1Pe 2:9). Christ works in heaven both as the king and high priest over this royal priesthood (his church). He makes intercession for us (Heb 7:24-25, Heb 8:1-2, Rom 8:34).

2 The Work of the Holy Spirit

• The Spirit's work toward salvation
  On the day of Pentecost, when Peter and the other apostles were preaching, "the Holy Spirit was giving them utterance" (Acts 2:4). The Spirit was teaching them what to say as Christ had promised (Jhn 14:26, Jhn 16:13, Lke 12:12, Acts 1:8). The Spirit was giving them revelation, and moving them to speak the message of salvation just as he had moved the prophets of old (2Pe 1:21).
• The Spirit's work following salvation
  Peter promised all who respond to the call of the gospel, "You shall receive the gift of the Holy Spirit" (Acts 2:38-39). Those who are saved "live in the Spirit" (1Pe 3:18, 1Pe 4:6). He is "given as a pledge of our inheritance" (Eph 1:13-14).

3 The Work of Man

• Man's work in the crucifixion
  Coming back now to the cross of Christ, we notice that Peter proclaims that Jesus was delivered up to be crucified "by the predetermined plan and foreknowledge of God". Peter further says, "You nailed [him] to a cross by the hands of godless men" (Acts 2:23). Notice the distinction Peter makes. The crucifixion of Christ was a work of man, but not under God. Whilst God anticipated the wicked work, he did not by any means purpose it. Rather, he purposed to deliver his Son to suffer it. He foreknew the wicked act, he predetermined the plan, but he did not command the wicked act to be done.
• Man's work in his own salvation
  Man can do nothing to gain his own forgiveness and salvation except what God commands him to do. When those convicted of sin asked, "What shall we do?" they were told what God required of them, "Repent and be baptized every one of you..." (Acts 2:37-38). When you work in obedience to the word of God, "it is God who is at work in you, both to will and to work for his good pleasure" (Php 2:12-13). This was what Peter had in mind when he wrote, "...baptism also saves us..." (1Pe 3:21). When man works for his own salvation, he does not work his own works, but rather the works of God or "the obedience of faith" (Jhn 6:27-29, Rom 1:5).