Cyril of Alexandria — as excerpted in the Catena Aurea on Luke 9:18-22

“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.”