Peter, the fisherman, was a man of action.  He liked making decisions and announcing what he intended to do.  So when Jesus told him that evil was lurking around him, and Satan wanted to destroy him (Luke 22:31-32), he thought that his own moral strength and courage would win through.  Peter wanted to prove that he was the best of all the disciples.  Even though Jesus had said that Satan's desire to ruin Peter was so strong that Jesus had to intervene, the arrogant disciple still protested that his loyalty to Jesus was rock firm.

Peter did not know, either the infinite depth of Jesus’ knowledge about him, or the infinite depravity to which he could sink.  But Jesus had it all in hand.  He knew the temptation pit into which Peter must fall to expose his pride and moral weakness – so that he could become useable for God’s kingdom.  Jesus also knew the much greater depth of His grace which would completely forgive the sinner and restore him to useful servanthood (John 21:15-19).
 
Jesus knew that all Peter's good intentions would come to nothing in a few hours.  He even pinpointed the exact time of day, and the number of times Peter would deny Him.  It was a shocking prophecy which probably meant little at the time it was spoken; however, after the cock crowed early in the morning, the catalogue of Peter's failure crashed into his heart and he realised that he had failed so badly (Mark 14:72).
 
Most people who follow Jesus really do want to serve Him to the best of their ability.  But we need to understand that, without His help, we are totally incapable of doing so.  The weakness of our human nature is a trap we keep falling into - however much we protest that we will never dishonour Jesus.  But our human weakness is not the end of the matter if we will only abandon our proud self-assertion and accept the help which the Lord wants to give us.  Promises to defeat Satan by ourselves may sound good, but will always fail.  However, those who start by admitting their weakness, and cry out to the Lord will always find the Lord's help is alongside (2 Corinthians 12:9).