9 For the Father being Very God begot the Son like Himself, Very God; not as teachers beget disciples, not as Paul says to some, For in Christ Jesus I begot you through the Gospel. For in this case he who was not a son by nature became a son by discipleship, but in the former case He was a Son by nature, a true Son. Not as you, who are to be illuminated, are now becoming sons of God: for you also become sons, but by adoption of grace, as it is written, But as many as received Him, to them gave He the right to become children of God, even to them that believe in His name: which were begotten not of blood, nor of the will of the flesh, nor of the will of man, but of God. And we indeed are begotten of water and of the Spirit, but not thus was Christ begotten of the Father. For at the time of His Baptism addressing Him, and saying, This is My Son, He did not say, “This has now become My Son,” but, This is My Son; that He might make manifest, that even before the operation of Baptism He was a Son.
Source: Catechetical Lectures (New Advent)