Acts chapter two records the first conversions to Christ after His resurrection. Jesus earlier prayed for himself, his apostles and all believers (John 17), then his suffering and death followed as he had predicted, saying that “repentance and forgiveness of sins” would be preached to all nations beginning in Jerusalem (Lk 24:46-48).
All arrows point to Acts chapter two. Devout Jews from every nation were gathered and heard the apostles preaching that Jesus had to die because of their sins (Ac 2:14-40; Rom 10:17).
They believed what they were taught about Jesus (Ac 2:37), that the one they had mercilessly killed was the Son of God. They then were told to surrender their lives in repentance, to change their minds and actions (Ac 26:20, 2:38, 42-47). Peter and the other apostles commanded the people to be immersed, buried in baptism like Jesus had been buried just a few days before, in order that their sins might be forgiven (Acts 2:38). Baptism is where the dead body of sin is buried, and then raised up to go a new direction in life with God (Rom 6:2-4). This was earlier taught by Jesus in his last words on earth, that a person must be baptized to be saved (Mk 16:16).
3,000 believers were added that day. They were known as “the church” (verse 47) and later in all cities, “churches of Christ” (Rom 16:16).