"Now, as I said before, the reason God's plan permitted Peter to sin was because he was to be entrusted with the whole people of God, and sinlessness added to his severity might have made him unforgiving toward his brothers and sisters. He fell into sin so that remembering his own fault and the Lord's forgiveness, he also might forgive others out of love for them. This was God's providential dispensation. He to whom the Church was to be entrusted, he, the pillar of the churches, the harbor of faith, was allowed to sin; Peter, the teacher of the world, was permitted to sin, so that having been forgiven himself he would be merciful to others."
John Chrysostom (AD 347-407), In SS. Petrum et Heliam: PG 50, 727-728
Chrysostom was the patriarch of Constantinople and was a masterful preacher, earning him the title of "the golden-mouthed."
This quote is simply stunning. What we mean for evil God uses for good. What we view as weak and frail - like Peter - God deigns to build His whole Church on.