-- Letter 53, 2-3
In response to Peter's confession of Christ's divine sonship (in Matthew 16), Christ confessed Peter as the rock of the Church, mysteriously and incredibly fusing God's own rock-solid strength with Peter's apostolic authority as well as with his rock-like confession of faith. Leaving aside for the moment the implications of the fact that Christ showed Peter a remarkable singularity of prominence among the Apostles, this quote emphasizes the more general principle that the perpetual solidity of the Church rests on God by means of an historical - that is, incarnate - episcopal succession.
Prayer. Lord, teach me what I should teach, teach me what I should hold fast.
-- Letter 166, 10