Peter's Answer

Ἀποκριθεὶς δὲ Σίμον Πέτρος, εἶπε· Σὺ εἶ ὁ Χριστὸς ὁ Υἰὸς τοῦ Θεοῦ τοῦ ζῶντος.   Οὐκ εἶπεν ὁ Πέτρος· Σὺ εἶ Χριστὸς, ἢ Υἰὸς τοῦ Θεοῦ· ἀλλὰ· Ὁ Χριστὸς ὁ Υἰὸς τοῦ Θεοῦ. Πολλοὶ μὲν γὰρ χριστοὶ κατὰ χάριν, καὶ υἰοθεσίας ἀξίωμα ἔχοντες, ἀλλὰ μόνος εἶς ὁ φύσει Υἰὸς τοῦ Θεοῦ. Διὸ μετὰ ἂρθμου ἔφη· Ὁ Χριστὸς ὁ Υἰὸς τοῦ Θεοῦ. Εἰπὼν δὲ αὐτὸν Υἰὸν τοῦ Θεοῦ τοῦ ζῶντος, δείκνυσιν ὅτι αὐτὸς ζωὴ ἦν, καὶ θάνατος αὐτοῦ οὐ κυριεύει. Κἂν γὰρ πρὸς ὀλιγον ἡ σὰρξ ἠσθένησεν ἀποθανοῠσα, ἀλλ' ἄνέστη, μὴ δυναμένου τοῦ ἐν αὐτῇ Λόγου ὑπὸ δεσμὰ θανάτου κρατεῖσθαι. Ἅγιος Κύριλλος Ἀλεξανδρείας, Ἐξὴγησις Εἰς Τὸ Κατὰ Ματθαιον Εὐάγγελιον

And Peter answered, saying, 'You are the Christ, the Son of the living God.' 1 Peter did not say, 'You are a Christ, a son of God,' but 'the Christ, the Son of God,' for many are the christs anointed by grace who are thus made worthy of adoption as sons, but only one is by nature the Son of God. Therefore with the article he said, 'The Christ, The Son of God.' And saying that He is the Son of the Living God, he shows Him to be life and that death has no dominion over Him. For it was permitted that for a little time the weak flesh die, but He rose, for it was not able, with the Word in it, to be kept bound by death. Saint Cyril of Alexandria, Commentary on Matthew, Fragment 1 Mt.16.16