When a scripture reading begins with “Now when they heard this…” it sort of begs the question of what it is they heard. Like when you find “Therefore” in scripture and ask, as you should, what is the therefore there for?
In this case the things that the people heard about were things about Jesus, specifically Acts 2:36, which reads, “Let the entire house of Israel know with certainty that God has made him both Lord and Messiah, this Jesus whom you crucified.”
Not “this Jesus, the Son of God” or “this Jesus” period, but “this Jesus, whom you crucified.” You did this, Peter says, pointing the finger at the very people who are beginning the church.
This gets at a strange tension in what it means to be a Christian. Yes, we are people who follow God, but we are also people who would probably nail our God to a cross again if we were given the chance—probably not knowing what we were doing. These people in Acts were the original big tent church, spirit-filled, Pentecostals of a sort. These, of all people, were ready to receive the body and blood of Christ in communion with the words “Given for you” or “Broken for you” or “Shed for you.” And yet, these were also the people who betrayed Jesus to be crucified. At least they were by association. To be a Christian is to be a person who is saved by Jesus even while we are people who killed Jesus in the first place. As with these people in Acts, that cuts us to the heart.
So we repent. We ask forgiveness. Not cheap forgiveness; not “because we got caught so we sort of feel bad about it” forgiveness, but real, tough forgiveness. The kind that sits with us. This is evident enough in what Peter tells us to do: Repent, be baptized, participate in the life of discipleship through teaching and visiting and eating together and praying together. All of this happens as a gift of God who, Peter says, calls us to him. God calls us; we respond by how we live.
This is really what the sacraments are about: God’s gifts pushing us to respond by living out of gratitude for the grace we have been given.