"He passed over his fall, and appointed him first of the Apostles; wherefore He said: ' 'Simon, Simon,' etc. (in Ps. cxxix. 2). God allowed him to fall, because He meant to make him ruler over the whole world, that, remembering his own fall, he might forgive those who should slip in the future. And that what I have said is no guess, listen to Christ Himself saying: 'Simon, Simon, etc.'"
