From Acts of the Apostles, by William S. Kurz, SJ, commenting on Acts 2:14a, 36-41
Listening to Peter’s speech, the people are cut to the heart—a sign that the Holy Spirit has convicted their consciences of sin and opened their hearts to believe Peter’s message. To their question what they are to do, Peter responds: “Repent and be baptized”—summing up the Christian call to conversion, which is grounded in Jesus’ own message, “Repent and believe in the gospel” (Mark 1:15).
Similar instructions will be given by Ananias at Paul’s conversion: “Now, why delay? Get up and have yourself baptized and your sins washed away, calling upon his name” (Acts 22:16). Even though one aspect or the other may not be explicit, throughout Acts the response that the gospel requires is threefold: faith, repentance, and baptism.
In the New Testament, forgiveness of sins is closely linked with repentance. God desires to forgive all sinners, but his respect for human freedom necessitates that we accept his forgiveness by admitting and repenting of our guilt. This parallels a fact of human experience, that when a wrong has been committed but not acknowledged and renounced, even if the person who suffered the wrong is willing to forgive, reconciliation remains incomplete.
Like the whole New Testament, Peter affirms that entrance into Christ’s Church is through baptism, at least for everyone after the initial 120 on whom the Spirit first descended. A useful comment is offered by Raymond Brown:
“Baptism as a public action is important. . . . Peter is portrayed as asking people to make a visible and verifiable profession of their acceptance of Jesus. This is tantamount to asking people to ‘join up.’ The basic Israelite concept is that God chose to save a people, and the renewal of the covenant on Pentecost has not changed that. There is a collective aspect to salvation, and one is saved as part of God’s people.”