Catena Aurea on John 18:15–18: John Chrysostom

Therefore did Divine Providence permit Peter first to fall, in order that he might be less severe to sinners from the remembrance of his own fall. Peter, the teacher and master of the whole world, sinned, and obtained pardon, that judges might thereafter have that rule to go by in dispensing pardon. For this reason I suppose the priesthood was not given to Angels; because, being without sin themselves, they would punish sinners without pity. Passible man is placed over man, in order that remembering his own weakness, he may be merciful to others.