36 But the faith of the Church, while confessing the only true God the Father, confesses Christ also. It does not confess Christ true God without the Father the only true God; nor the Father the only true God without Christ. It confesses Christ true God, because it confesses the Father the only true God. Thus the fact that God the Father is the only true God constitutes Christ also true God. The Only-begotten God suffered no change of nature by His natural birth: and He Who, according to the nature of His divine origin was born God from the living God, is, by the truth of that nature, inalienable from the only true God.
Thus there follows from the true divine nature its necessary result, that the outcome of true divinity must be a true birth, and that the one God could not produce from Himself a God of a second kind. The mystery of God consists neither in simplicity, nor in multiplicity: for neither is there another God, Who springs from God with qualities of His own nature, nor does God remain as a single Person, for the true birth of the Son teaches us to confess Him as Father. The begotten God did not, therefore, lose the qualities of His nature: He possesses the natural power of Him, Whose nature He retains in Himself by a natural birth.
The divinity in Him is not changed, or degenerate, for if His birth had brought with it any defect, it would more justly cast upon the Nature, through which He came into being, the reflection of having failed to implant in its offspring the properties of itself. The change would not degrade the Son, Who had passed into a new substance by birth, but the Father, Who had been unable to maintain the constancy of His nature in the birth of the Son, and had brought forth something external and foreign to Himself.
Source: On the Trinity (New Advent)