39 Let us therefore cite every example of a statement of the faith made by an Apostle. All of them, when they confess the Son of God, confess Him not as a nominal and adoptive Son, but as Son by possession of the Divine nature. They never degrade Him to the level of a creature, but assign Him the splendour of a true birth from God. Let John speak to us, while he is waiting, just as he is, for the coming of the Lord; John, who was left behind and appointed to a destiny hidden in the counsel of God, for he is not told that he shall not die, but only that he shall tarry.
Let him speak to us in his own familiar voice:— No one has seen God at any time, except the Only-begotten Son, Which is in the bosom of the Father. It seemed to him that the name of Son did not set forth with sufficient distinctness His true Divinity, unless he gave an external support to the peculiar majesty of Christ by indicating the difference between Him and all others. Hence he not only calls Him the Son, but adds the further designation of the Only-begotten, and so cuts away the last prop from under this imaginary adoption. For the fact that He is Only-begotten is proof positive of His right to the name of Son.
Source: On the Trinity (New Advent)