Luke 22:31-32 (NKJV)
And the Lord said, "Simon, Simon! Indeed, Satan has asked for you, that he may sift you as wheat. But I have prayed for you, that your faith should not fail; and when you have returned to Me, strengthen your brethren."
Like he did with Job, Satan accuses Peter and the disciples by saying that if God will allow him to test them with trials and difficulties that they will fall away. God allows it and Jesus says something marvelous, "I have prayed for you" and then He prophesies that they will (when not if) return and strengthen the others.
Jesus always draws his own back to Himself; he will not let them fail. I have noticed that before I came to trust in the Lord, I would flounder in sin for a long time. Now, I feel the tug of my Lord drawing me back even after a short while. The reason we don't fail the test and fall away is not because of our willpower, but because Jesus intercedes for us.
Jesus died for the world but Jesus prays for his own.
Hebrews 7:25 (NKJV)
Therefore He is also able to save to the uttermost those who come to God through Him, since He always lives to make intercession for them.