Twice, Peter had already denied that he knew Jesus (Luke 22:56-58).  Now the third denial aggressively left his lips.  He was confronted by a man who had seen him with Jesus in the Garden of Gethsemane. John's account says, "One of the high priest’s servants, a relative of the man whose ear Peter had cut off, challenged him, ‘Didn’t I see you with him in the garden?’" (John 18:26).  

It is obvious to us that Peter, the trainee apostle, was now well out of control.  Having boasted that he would never deny Jesus (Luke 22:33-34) and having failed to watch and pray with Jesus in the Garden of Gethsemane (Luke 22:40-46), Peter was operating in the strength of his own flesh.  His proud promises evaporated leaving him to try to save his own skin by pretending that he did not know Jesus.  And even more foolishly, willingly placed himself under God's curse if he was lying - Mark wrote that Peter began to call down curses on himself and swore to them (Mark 14:71).
 
At that moment the cock crowed.  It was just then that Jesus emerged into the courtyard, pausing briefly to make eye contact with Peter.  It sounded an alarm within Peter.  But it wasn't the dawn chorus which pierced Peter's heart.  It was the words of Jesus who had accurately prophesied Peter's disgraceful behaviour only hours before (Mark 14:27-30).  Peter now realised that Jesus knew exactly what was in his heart, and how it would manifest itself (Psalm 139:4).  So, after three years of ministry training, Peter realised that he was certainly not good enough to represent Jesus to the world.  Despite Peter’s protestations, Jesus knew Peter so much better than he knew himself … and He was on the way to Calvary to pay for all Peter's sins, and all of ours (www.crosscheck.org.uk).
 
Although religion encourages people to pretend that they are good, true faith in Christ can only start when we realise that we are not.  True repentance is not only an admission of our wrongdoing, but the awareness that Jesus knows the deep inner hidden wickedness of our hearts.  Too many people who call themselves Christians are trying to convince God that they deserve to be in His kingdom: but that approach, before the all-seeing eye of God, is a delusion (Matthew 7:21-23).  Although Peter's failure was a shock to him, it did not surprise Jesus.  However, it was essential for Peter to realise that he could not save himself, if he was to receive the grace of salvation from Jesus.  What was true for him is true for each of us.