We have to face those parts of our lives which we have hidden away, chosen to ignore or down played as “well everyone does it” before we can know what to ask God to heal. We act in ignorance. We make bad choices when we don’t have all the information and sometimes we make bad decisions anyway.
Peter is guilt ridden and lost until he faced this truth about himself. He was not able to accept the healing from Jesus until he was willing to face his own faults and admit them to God. Peter is trying to help the crowd see this too. Peter’s willingness to see his own faults and failures is what makes him credible and relatable to those he is trying to convince.