7 Let us then bring forward the words, having reminded you now, as I exhorted you at the first, earnestly to attend to what is said. What then does this Evangelist say immediately on his outset?
“In the beginning was the Word, and the Word was with God.” Do you see the great boldness and power of the words, how he speaks nothing doubting nor conjecturing, but declaring all things plainly? For this is the teacher's part, not to waver in anything he says, since if he who is to be a guide to the rest require another person who shall be able to establish him with certainty, he would be rightly ranked not among teachers, but among disciples.
But if any one say, “What can be the reason that he has neglected the first cause, and spoken to us at once concerning the second?” we shall decline to speak of “first” and “second,” for the Divinity is above number, and the succession of times. Wherefore we decline these expressions; but we confess that the Father is from none, and that the Son is begotten of the Father. Yes, it may be said, but why then does he leave the Father, and speak concerning the Son? Why? Because the former was manifest to all, if not as Father, at least as God; but the Only-Begotten was not known; and therefore with reason did he immediately from the very beginning hasten to implant the knowledge of Him in those who knew Him not.
Besides, he has not been silent as to the Father in his writings on these points. And observe, I beg of you, his spiritual wisdom. He knows that men most honor the eldest of beings which was before all, and account this to be God. Wherefore from this point first he makes his beginning, and as he advances, declares that God is, and does not like Plato assert, sometimes that He is intellect, sometimes that He is soul; for these things are far removed from that divine and unmixed Nature which has nothing common with us, but is separated from any fellowship with created things, I mean as to substance, though not as to relation.
And for this reason he calls Him “The Word.” For since he is about to teach that this “Word” is the only-begotten Son of God, in order that no one may imagine that His generation is passible, by giving Him the appellation of “The Word,” he anticipates and removes beforehand the evil suspicion, showing that the Son is from the Father, and that without His suffering (change).
Source: Homilies on the Gospel of John (New Advent)