Luke 9:20

Thou seest the skilfulness of the question. He did not at once say, "Whom do ye say that I am?" but refers to the rumour of those that were without, that having rejected it, and shewn it to be unsound, He may then bring them back to the true opinion. Which also happened: for when the disciples had said, "Some John the Baptist, and others Elijah, and others, that some prophet of those in old time has risen up;" He said to them, "But ye, whom do ye say that I am?" Oh! how full of meaning is that "ye!" He separates them from all others, that they may also avoid their opinions, and not conceive an unworthy idea of Him, nor entertain confused and wavering thoughts, themselves too imagining that John had risen again, or one of the prophets. Ye therefore, He says, who have been chosen; who by My decree have been called to the apostleship; who are the witnesses of My miracles; whom do ye say that I am? First before the rest Peter again springs forth, and makes himself the mouthpiece of the whole company, pouring forth the expression of love to God, and giving utterance to a correct and faultless confession of faith in Him, saying, "The Christ of God." The disciple is unerring: a thoroughly intelligent explainer of the mystery. For he does not simply say, that He is a Christ of God; but "the Christ" rather: for there are many who have been called "Christ," from having in various ways been anointed of God. For some have been anointed as kings; and some as prophets; while others, having received salvation by That Christ Who is the Saviour of all, even we ourselves, obtain the appellation of christ, as having been anointed by the Holy Ghost. […]

In this one name there is the expression both of His divinity and incarnation, and the belief of His passion. He has therefore comprehended every thing, having expressed both the nature and the name wherein is all virtue. (summa virtutum)

But mark the subtle skill of the question. For he directs them first to the praises of strangers, that having overthrown these, He might beget in them the right opinion. So when the disciples had given the opinion of the people, He asks them their own opinion; as it is added, And He said unto them, Whom say ye that I am? How marked is ye! He excludes them from the other, that they may avoid their opinions; as if He said, Ye who by my decree are called to the Apostleship, the witnesses of my miracles, whom do ye say that I am? But Peter anticipated the rest, and becomes the mouthpiece of the whole company, and launching forth into the eloquence of divine love, utters the confession of faith, as it is added, Peter answering said, The Christ of God. He says not merely that He was Christ of God, but now He uses the article. Hence it is in the Greek, τὸν χριστόν. For many divinely accounted persons are in diverse ways called Christs, for some were anointed kings, some prophets. But we through Christ have been anointed by the holy Spirit, have obtained the name of Christ. But there is only one who is the Christ of God and the Father, He alone as it were having His own Father who is in heaven. And so Luke agrees indeed in the same opinion as Matthew, who relates Peter to have said, Thou art Christ, the Son of the living God, but speaking briefly Luke says that Peter answered, the Christ of God. But we must observe, that Peter most wisely confessed Christ to be one, against those who presumed to divide Immanuel into two Christs. For Christ did not enquire of them, saying, Whom do men say the divine Word is? but the Son of man, whom Peter confessed to be the Son of God. […]

There are many who have been called Christ, from having in various ways been anointed by God. Some have been anointed as kings. Some have been anointed as prophets. They have been so called because they have been anointed. But he who is God the Father's Christ is One, and One only. Not as though we are christs, and not God's christs, belonging to some other person. There is only one Christ, because he and he alone has as his Father God who is in heaven. Since, therefore, most wise Peter, confessing the faith correctly and without error, said, "the Christ of God," it is plain that Peter referred to Jesus as God. For Peter confessed Jesus to be God's sole Christ, distinguishing him from those to whom the appellation generally belongs. For though he be by nature God and shone forth inexpressibly from God the Father as his only-begotten Word, yet he became flesh according to the Scripture.

For even Peter would not have been able to say, "Thou art the Christ," unless he had beforehand heard and believed Moses and the prophets, by whom alone Christ had been hitherto announced.