What does this verse have to do with our discussion? Peter is preaching repentance to the nation of Israel, not salvation of an individual soul.
37 Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?
If this is true, and we crucified our Messiah, what shall we do? The question is not, what shall we do to be saved? That question is found later in Acts 16.
30 And brought them out, and said, Sirs,
what must I do to be saved?
31 And they said,
Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house.
Paul does not tell him, don't worry friend, you're already saved, now just believe.