And Jesus said to them, “You will all fall away, for it is written, ‘I will strike the shepherd, and the sheep will be scattered.’ 28 But after I am raised up, I will go before you to Galilee.” 29 Peter said to him, “Even though they all fall away, I will not.” 30 And Jesus said to him, “Truly, I tell you, this very night, before the rooster crows twice, you will deny me three times.” 31 But he said emphatically, “If I must die with you, I will not deny you.” And they all said the same. – Mark 14:27-31
Peter said in Mark 14:31, “I will never disown you.” Peter denied even the idea that he could fall away from Jesus, he denied that he could ever sin against his Lord, and because he denied this he took the first step to falling away from Jesus.
It sounds so odd, but to finally admit that you are capable of turning your back on God, is the first defense against turning your back on God.