II. The Creed takes up St. Peter's confession as the fundamental doctrine of the Church
In that rule of Faith, dearly-beloved, which we have received in the very beginning of the Creed, on the authority of apostolic teaching, we acknowledge our Lord Jesus Christ, whom we call the only Son of God the Father Almighty, to be also born of the Virgin Mary by the Holy Ghost. Nor do we reject His Majesty when we express our belief in His crucifixion, death, and resurrection on the third day. For all that is God's and all that is Man's are simultaneously fulfilled by His Manhood and His Godhead, so that in virtue of the union of the Passible with the Impassible, His power cannot be affected by His weakness, nor His weakness overcome by His power.
And rightly was the blessed Apostle Peter praised for confessing this union, who when the Lord was inquiring what the disciples knew of Him, quickly anticipated the rest and said, “You are Christ, the Son of the living God.” And this assuredly he saw, not by the revelation of flesh or blood, which might have hindered his inner sight, but by the very Spirit of the Father working in his believing heart, that in preparation for ruling the whole Church he might first learn what he would have to teach, and for the solidification of the Faith, which he was destined to preach, might receive the assurance, “You are Peter, and upon this rock I will build My Church, and the gates of hell shall not prevail against it.”
The strength, therefore, of the Christian Faith, which, built upon an impregnable rock, fears not the gates of death, acknowledges the one Lord Jesus Christ to be both true God and true Man, believing Him likewise to be the Virgin's Son, Who is His Mother's Creator: born also at the end of the ages, though He is the Creator of time: Lord of all power, and yet one of mortal stock: ignorant of sin, and yet sacrificed for sinners after the likeness of sinful flesh.
Source: Sermons (New Advent)