16 And these indeed are testimonies bearing on the incarnation of the Word; and there are also very many others. But let us also look at the subject in hand—namely, the question, brethren, that in reality the Father's power, which is the Word, came down from heaven, and not the Father Himself. For thus He speaks: “I came forth from the Father, and have come.” Now what subject is meant in this sentence, “I came forth from the Father,” but just the Word? And what is it that is begotten of Him, but just the Spirit, that is to say, the Word?
But you will say to me, How is He begotten? In your own case you can give no explanation of the way in which you were begotten, although you see every day the cause according to man; neither can you tell with accuracy the economy in His case. For you have it not in your power to acquaint yourself with the practised and indescribable art (method) of the Maker, but only to see, and understand, and believe that man is God's work. Moreover, you are asking an account of the generation of the Word, whom God the Father in His good pleasure begot as He willed.
Is it not enough for you to learn that God made the world, but do you also venture to ask whence He made it? Is it not enough for you to learn that the Son of God has been manifested to you for salvation if you believe, but do you also inquire curiously how He was begotten after the Spirit? No more than two, in truth, have been put in trust to give the account of His generation after the flesh; and are you then so bold as to seek the account (of His generation) after the Spirit, which the Father keeps with Himself, intending to reveal it then to the holy ones and those worthy of seeing His face?
Rest satisfied with the word spoken by Christ, viz., “That which is born of the Spirit is spirit,” just as, speaking by the prophet of the generation of the Word, He shows the fact that He is begotten, but reserves the question of the manner and means, to reveal it only in the time determined by Himself. For He speaks thus: “From the womb, before the morning star, I have begotten You.”
Source: Against the Heresy of Noetus (New Advent)