15 Jesus saith to them: But whom do you say that I am? 16 Simon Peter answered and said: Thou art Christ, the Son of the living God.
Chrysostom: When the Lord enquires concerning the opinion of the multitudes, all the disciples answer; but when all the disciples are asked, Peter as the mouth and head of the Apostles answers for all.
Origen: He added what was more, “the Son of the living God,” who had said by his Prophets, “I live, saith the Lord” [Ezek 33:11]. And therefore was He called the living Lord, but in a more especial manner as being eminent above all that had life; for He alone has immortality, and is the fount of life, wherefore He is rightly called God the Father; for He is life as it were flowing out of a fountain, who said, “I am the life” [John 14:6].
Jerome: He calls Him “the living God,” in comparison of those gods who are esteemed gods, but are dead; such, I mean, as Saturn, Jupiter, Venus, Hercules, and the other monsters of idols.
Hilary: This is the true and unalterable faith, that from God came forth God the Son, who has eternity out of the eternity of the Father.
Rabanus: The Lord Himself puts forward the lowliness of the humanity which He had taken upon Him, while His disciple shews us the excellence of His divine eternity.
17 And Jesus answering, said to him: Blessed art thou, Simon Bar-Jona: because flesh and blood hath not revealed it to thee, but my Father who is in heaven.
Hilary: This confession of Peter met a worthy reward, for that he had seen the Son of God in the man.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. I (London: Rivington, 1842).