3 We will offer later an explanation of these texts in the words of the Gospels and Epistles themselves. But first we hold it right to remind the members of our common faith, that the knowledge of the Eternal is presented in the same confession which gives eternal life. He does not, he cannot know his own life, who is ignorant that Christ Jesus was very God, as He was very man. It is equally perilous, whether we deny that Christ Jesus was God the Spirit, or that He was flesh of our body: Every one therefore who shall confess Me before men, him will I also confess before My Father which is in Heaven.
But whosoever shall deny Me before men, him will I also deny before My Father which is in heaven</em>. So said the Word made flesh; so taught the man Jesus Christ, the Lord of majesty, constituted Mediator in His own person for the salvation of the Church, and being in that very mystery of Mediatorship between men and God, Himself one Person, both man and God. For He, being of two natures united for that Mediatorship, is the full reality of each nature; while abiding in each, He is wanting in neither; He does not cease to be God because He becomes man, nor fail to be man because He remains for ever God.
This is the true faith for human blessedness, to preach at once the Godhead and the manhood, to confess the Word and the flesh, neither forgetting the God, because He is man, nor ignoring the flesh, because He is the Word.
Source: On the Trinity (New Advent)