Why? Peter denied Jesus three times written about in Matthew 26, Mark 14 and Luke 22, he was afraid that they might take him also as they were beating our Lord. That same Peter in Acts 2 stands up and tells these same men of Israel to heed his words and repent.
Act 2:22-23 “Men of Israel, hear these words: Jesus of Nazareth, a man attested by God to you by miracles, wonders, and signs which God did through Him in your midst, as you yourselves also know---- Him being delivered by the determined purpose and foreknowledge of God, you (emphasis added) have taken by lawless hands, have crucified, and put to death.”
This once timid man now after Christ has risen has boldness to stand and speak with authority for repentance. The disciples once cowards that did not even bury their leader, but left it to Joseph of Arimathea part of the Sanhedrin, these same men turned into martyrs for Christ. They would willingly give their very lives and die for Christ, because they believed He was the Son of God. 
First you must believe that He is!
Sunday is on its way!!
1. Book: First You Must Believe That He Is! by Sherry Gorslin