Theophylact of Ohrid — as excerpted in the Catena Aurea on Luke 22:31-34

“For albeit thou art for a time shaken, yet thou boldest stored up, a seed of faith; though the spirit has shed its leaves in temptation, yet the root is firm. Satan then seeks to harm thee, because he is envious of my love for thee, but notwithstanding that I have prayed for thee, thou shalt fall. Hence it follows, And when thou art converted, strengthen thy brethren. As if He says, After that thou hast wept and repented thy denial of Me, strengthen thy brethren, for I have deputed thee to be the head of the Apostles. For this befits thee who art with Me, the strength and rock of the Church. And this must be understood not only of the Apostles who then were, but of all the faithful who were about to be, even to the end of the world; that none of the believers might despair, seeing that Peter though an Apostle denied his Lord, yet afterwards by penitence obtained the high privilege of being the Ruler (ἐπιστάτης) of the world.”