There was nothing very great in our Lord saying whose son he was, for our Lord knew the names of all His saints, having predestinated them before the foundation of the world. But it was a great thing for our Lord to change his name from Simon to Peter. Peter is from petra, rock, which rock is the Church: so that the name of Peter represents the Church. And who is safe, unless he build upon a rock? Our Lord here rouses our attention: for had he been called Peter before, we should not have seen the mystery of the Rock, and should have thought that he was called so by chance, and not providentially. God therefore made him to be called by another name before, that the change of that name might give vividness to the mystery.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Tr. vii. c. 14