One reason that Peter was so credible and remains so credible to this very day is that we can all somehow relate to his story. He screwed up, and he screwed up bad. There was nothing he could do to fix himself except throw himself at the mercy of Jesus, which is exactly what he did. Jesus forgave his sin, and from that point on, Peter looked back at those dark moments as bright ones, not because of anything that he did, but because of what the Lord did for him. Peter experienced a healing of memory. He no longer remembered his sins as sins committed, but as sins confessed and forgiven. It made all the difference in the world. And it still does.
Sins Committed vs. Sins Confessed and Forgiven