4 Again, I say, on hearing of a Son, understand it not merely in an improper sense, but as a Son in truth, a Son by nature, without beginning; not as having come out of bondage into a higher state of adoption, but a Son eternally begotten by an inscrutable and incomprehensible generation. And in like manner on hearing of the First-born, think not that this is after the manner of men; for the first-born among men have other brothers also. And it is somewhere written, Israel is My son, My first-born. But Israel is, as Reuben was, a first-born son rejected: for Reuben went up to his father's couch; and Israel cast his Father's Son out of the vineyard, and crucified Him.
To others also the Scripture says, You are the sons of the Lord your God: and in another place, I have said, You are gods, and you are all sons of the Most High. I have said, not, “I have begotten.” They, when God so said, received the sonship, which before they had not: but He was not begotten to be other than He was before; but was begotten from the beginning Son of the Father, being above all beginning and all ages, Son of the Father, in all things like to Him who begot Him, eternal of a Father eternal, Life of Life begotten, and Light of Light, and Truth of Truth, and Wisdom of the Wise, and King of King, and God of God, and Power of Power.
Source: Catechetical Lectures (New Advent)