36 A belief that the Son of God is Son in name only and not in nature, is not the faith of the Gospels and of the Apostles. If this be a mere title, to which adoption is His only claim; if He be not the Son in virtue of having proceeded forth from God, whence, I ask, was it that the blessed Simon Bar-Jona confessed to Him, You are the Christ, the Son of the living God ? Because He shared with all mankind the power of being born as one of the sons of God through the sacrament of regeneration?
If Christ be the Son of God only in this titular way, what was the revelation made to Peter, not by flesh and blood, but by the Father in heaven? What praise could he deserve for making a declaration which was universally applicable? What credit was due to Him for stating a fact of general knowledge? If He be Son by adoption, wherein lay the blessedness of Peter's confession, which offered a tribute to the Son to which, in that case, He had no more title than any member of the company of saints?
The Apostle's faith penetrates into a region closed to human reasoning. He had, no doubt, often heard, He that receives you receives Me, and He that receives Me receives Him that sent Me. Hence he knew well that Christ had been sent; he had heard Him, Whom he knew to have been sent, making the declaration, All things are delivered unto Me of the Father, and no one knows the Son but the Father, neither knows any one the Father save the Son. 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 of '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. For though Christ, while dwelling in the body, had avowed Himself to be the Son of God, yet now for the first time the Apostle's faith had recognised 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.
And this is the rock of confession whereon 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 honour, 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.
Source: On the Trinity (New Advent)