2 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, and says, “You are the Christ, the Son of the living God.”
What then says Christ? “Blessed are you, Simon Barjona, for flesh and blood has not revealed it unto you.”
Yet surely unless he had rightly confessed Him, as begotten of the very Father Himself, this were no work of revelation; had he accounted our Lord to be one of the many, his saying was not worthy of a blessing. Since before this also they said, “Truly He is Son of God,” those, I mean, who were in the vessel after the tempest, which they saw, and were not blessed, although of course they spoke truly. For they confessed not such a Sonship as Peter, but accounted Him to be truly Son as one of the many, and though peculiarly so beyond the many, yet not of the same substance.
And Nathanael too said, “Rabbi, You are the Son of God, You are the King of Israel;” and so far from being blessed, he is even reproved by Him, as having said what was far short of the truth. He replied at least, “Because I said unto you, I saw you under the fig-tree, do you believe? You shall see greater things than these.”
Why then is this man blessed? Because he acknowledged Him very Son. Wherefore you see, that while in those former instances He had said no such thing, in this case He also signifies who had revealed it. That is, lest his words might seem to the many (because he was an earnest lover of Christ) to be words of friendship and flattery, and of a disposition to show favor to Him, he brings forward the person who had made them ring in his soul; to inform you that Peter indeed spoke, but the Father suggested, and that you might believe the saying to be no longer a human opinion, but a divine doctrine.
And wherefore does He not Himself declare it, nor say, “I am the Christ,” but by His question establish this, bringing them in to confess it? Because so to do was both more suitable to Him, yea necessary at that time, and it drew them on the more to the belief of the things that were said.
Do you see how the Father reveals the Son, how the Son the Father? For “neither knows any man the Father,” says He, “save the Son, and he to whomsoever the Son will reveal Him.” It cannot therefore be that one should learn the Son of any other than of the Father; neither that one should learn the Father of any other than of the Son. So that even hereby, their sameness of honor and of substance is manifest.
Source: Homilies on the Gospel of St. Matthew (New Advent)