And when they had sung a hymn, they went out to the Mount of Olives. Then
Jesus said to them, “You will all fall away because of me this night. For it is
written, ‘I will strike the shepherd, and the sheep of the flock will be
scattered.’ But after I am raised up, I will go before you to Galilee.” Peter
answered him, “Though they all fall away because of you, I will never fall
Jesus said to him, “Truly, I tell you, this very night, before the rooster crows, you will deny me three times.” Peter said to him, “Even if I must die with you, I will not deny you!” And all the disciples said the same.
(Matthew 26:30-35 ESV)
He that now abhors the thoughts of such and such a thing, if he once enters into temptation will find his heart inflamed towards it, and all contrary reasonings overborne and silenced. He will deride his former fears, cast out his scruples, and condemn the consideration that he lived upon. Little did Peter think he should deny and forswear his Master so soon as ever he was questioned whether he knew him or [not]. It was no better when the hour of temptation came; all resolutions were forgotten, all love to Christ buried; the present temptation closing with his carnal fear carried all before it….
‘Though all men should deny thee,’ [says Peter,] ‘I will not. Shall I do this evil? It cannot be.’ All the arguments that are suited to give check to the heart in such a condition are mustered up… ‘What! deny my Master, the Son of God, my Redeemer, who loves me? Can such ingratitude, unbelief, rebellion, befall me? I will not do it.’ Shall, then, a man in it that his heart will be steadfast? Let the wise man answer: ‘He that trusteth in his own heart is a fool.’”
From John Owen's Of Temptation