Acts 2:38 Then Peter said to them, “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit.
Though forgiveness of sin could never be bought by either repentance or by baptism, Peter lists these because of their significance. It is the fact that Jesus purchased our sin debt with his blood that is important.
We are the ones that have to show that we know what it means, and what it meant. When we repent we are showing that we do trust Christ to forgive us and when we come forward to be baptised we are showing others that we acknowledge what Christ did for us.
We need to show God and others that we do trust in Him.