"But in asking whether the Son of Man would find faith on earth at his coming, the Lord was speaking of perfect faith. That kind of faith is indeed hardly to be found on earth. Look at God's Church: it is full of people. Who would come here if faith were non-existent? But who would not move mountains if that faith were present in full measure? Mark the apostles: they would never have left everything they possessed and spurned worldly ambition to follow the Lord unless their faith had been great; and yet that faith of theirs could not have been perfect, otherwise they would not have asked the Lord to increase it."
Augustine of Hippo (AD 354-430), Sermon 115
Augustine makes a good point, albeit obliquely, in the first paragraph. Was Jesus’ prayer for Peter hypothetical, a mere rhetorical flourish before the final bloody curtain of Calvary? Or was it a genuine prayer? Assuming the latter -- most certainly -- how can we deny the reality of which the prayer worked against, to wit, the FAILURE of faith? Is there not an ultimate point of sinful regression in faith that demands something at least conceptually (if not verbally) like "mortal sin"? I'm reminded of Romans 6 and Galatians 5 and Luke 8 and Revelation 22. Grist for the prayer mill.