This text is very blunt yet implies so much. This small phrase is increasingly rich nuggets worth taking to heart. First, Jesus forewarns Simon of how satan has to ask permission to "enter him" (Lk 22:3) and implies that God has granted that access. This, to me, was the first sign that God knew that Simon's faith was authentic enough to handle the fall Simon was about to encounter. God is the author of our faith and because only He knows who His "own" are, He is able to determine what we can handle from the tempter. Second, Jesus tells Simon that He has prayed for him. Wow, what an encouragement to be prayed for by Jesus, himself. The Son sits at the right hand of the Father, interceding for His children because we fight our battles from victory, not for victory.
Finally, this last statement is monumental to me and two-fold in meaning. Jesus tells Simon when, get that...he didn't say if...He said "when"... you have turned back. He is referencing that first point again, that He has validated Simon's faith. He is forecasting that Simon will in fact recover or be redeemed (if you will) from his fall. That satan will not completly destroy him, but just as he entered, he will leave. Then He challenges Simon to learn from his fall and in confidence of full forgiveness, turn and use his fall to help others who are subject and will surely face the same temptation.
What a powerful message from Jesus. He has confidence and assurance in His kids, and even when He gives the enemy authority (demonstrating His power and victory over death), He intends to fully restore us for the purpose of strengthening our fellow believers. Forgiveness and grace are woven through that tapestry of love. This should be the model with which we treat our brothers and sisters who fall, disappoint us, or even waver for a season. The same forgiveness, grace and love Jesus offered Simon through these words, should be mirrored in our behavior towards those whom instead we turn our noses up at, or gossip about sneering "I can't beleive they...", or those we reject and cut out of our lives due to a deed that left us rejected or betrayed.
~Deep Waters