I. Peter's confession shown to lead up to the Transfiguration
The Gospel lesson, dearly-beloved, which has reached the inner hearing of our minds through our bodily ears, calls us to the understanding of a great mystery, to which we shall by the help of God's grace the better attain, if we turn our attention to what is narrated just before.
The Saviour of mankind, Jesus Christ, in founding that faith, which recalls the wicked to righteousness and the dead to life, used to instruct His disciples by admonitory teaching and by miraculous acts to the end that He, the Christ, might be believed to be at once the Only-begotten of God and the Son of Man. For the one without the other was of no avail to salvation, and it was equally dangerous to have believed the Lord Jesus Christ to be either only God without manhood, or only man without Godhead, since both had equally to be confessed, because just as true manhood existed in His Godhead, so true Godhead existed in His Manhood. To strengthen, therefore, their most wholesome knowledge of this belief, the Lord had asked His disciples, among the various opinions of others, what they themselves believed, or thought about Him: whereat the Apostle Peter, by the revelation of the most High Father passing beyond things corporeal and surmounting things human by the eyes of his mind, saw Him to be Son of the living God, and acknowledged the glory of the Godhead, because he looked not at the substance of His flesh and blood alone; and with this lofty faith Christ was so well pleased that he received the fullness of blessing, and was endued with the holy firmness of the inviolable Rock on which the Church should be built and conquer the gates of hell and the laws of death, so that, in loosing or binding the petitions of any whatsoever, only that should be ratified in heaven which had been settled by the judgment of Peter.
Source: Sermons (New Advent)