What then is this truth, which the Father now reveals to Peter, which receives the praise of a blessed confession? It cannot have been that the names ‘Father’ and ‘Son’ were novel to him. He had heard them often. Yet he speaks words which the tongue of man had never framed before: “You are the Christ, the Son of the living God” (Mt 16:16). For though Christ, while dwelling in the body, had declared Himself to be the Son of God, yet now for the first time the Apostle’s faith had recognized in Him the presence of the divine nature. Peter is praised not merely for his tribute of adoration, but for his recognition of the mysterious truth; for confessing not Christ only, but Christ the Son of God. It would clearly have sufficed for a payment of reverence, had he said, “You are the Christ,” and nothing more. But it would have been a hollow confession, had Peter only hailed Him as Christ, without confessing Him the Son of God. And so his words “You are” declare that what is asserted of Him is strictly and exactly true to His nature. Next, the Father’s utterance, “This is My Son,” had revealed to Peter that he must confess “You are the Son of God,” for in the words “This is,” God the Revealer points Him out, and the response, “You are,” is the believer’s welcome to the truth.
This is the rock of confession on which the Church is built. But the perceptive faculties of flesh and blood cannot attain to the recognition and confession of this truth. It is a mystery, divinely revealed, that Christ must be not only named, but believed, the Son of God. Was it only the divine name; was it not rather the divine nature that was revealed to Peter? If it were the name, he had heard it often from the Lord, proclaiming Himself the Son of God. What honor, then, did he deserve for announcing the name? No, it was not the name; it was the nature, for the name had been repeatedly proclaimed.
This faith is the foundation of the Church. Through this faith the gates of hell cannot prevail against her. This is the faith which has the keys of the kingdom of heaven. Whatever this faith shall have loosed or bound on earth shall be loosed or bound in heaven. This faith is the Father’s gift by revelation; even the knowledge that we must not imagine a false Christ, a creature made out of nothing, but must confess Him the Son of God, truly possessed of the divine nature.
(Hilary of Poitiers, On the Trinity, 6.36-37; source: the Rev. Dr. Scott Murray, “Memorial Moments,” January 15, 2018)