And Simon Peter answered, You are the Christ, the Son of the living God.
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Cyril of Alexandria
Peter did not say “you are a Christ” or “a son of God” but “the Christ, the Son of God.” For there are many christs by grace, who have attained the rank of adoption [as sons], but [there is] only one who is by nature the Son of God. Thus, using the definite article, he said, the Christ, the Son of God. And in calling him Son of the living God, Peter indicates that Christ himself is life and that death has no authority over him. And even if the flesh, for a short while, was weak and died, nevertheless it rose again, since the Word, who indwelled it, could not be held under the bonds of death.
Did the Lord not know what people called him? But by questioning he brought forth the conviction of the apostle Peter and left for us in the future a strong affirmation of faith. For the Lord questioned not only Peter but all the apostles when he said, “Who do you say that I am?” Yet one on behalf of all answered the King, who is in due time to judge the whole world. He is God, both God and man. How miserable does this make those who are false teachers and strangers now, and to be judged in eternity. If Christ is the Son of God, by all means he is also God. If he is not God, he is not the Son of God. But since he himself is the Son, and as the Son takes up all things from the Father, let us hold this same one inseparably in our heart because there is no one who escapes his hand.
Simon Peter answering. As Simon Peter had been constituted the first in the college of apostles, (Matthew x. 2.) and therefore surpasseth the others in dignity as much as in zeal, without hesitation, and in the name of all, he answers: thou art the Christ, the Redeemer promised to the world, not a mere man, not a mere prophet like other prophets, but the true and natural Son of the living God. Thus Sts. Chrysostom, Cyril, Ambrose, Augustine, and Tirinus. When our Saviour inquired the opinion of the vulgar, all the apostles answered; but when he asks their opinion of him, Peter, as the mouth of the rest, and head of the whole college, steps forth, and prevents the others. (St. Chrysostom, hom. lv.)
Tu es Christus, filius Dei vivi; or, as it is in the Greek, o christos, o uios; The Christ, the Son, the Christ formerly promised by the law and the prophets, expected and desired by all the saints, the anointed and consecrated to God: o uios, the Son, not by grace only, or an adoptive filia...
What then says the mouth of the apostles, Peter, the ever fervent, the leader of the apostolic choir? When all are asked, he answers. And whereas when He asked the opinion of the people, all replied to the question; when He asked their own, Peter springs forward, and anticipates them.
. Once again Peter leaps forward with fervor and confesses that He is truly the Son of God. He did not say, "Thou art the anointed one, a son of God," without the article "the," but with the article, "the Son," that is, He Who is the One and the Only, not a son by grace, but He Who is begotten of the same essence as the Father. For there were also many other christs, anointed ones, such as all the priests and kings; but the Christ, with the article, there is but One.