20 For godliness it suffices you to know, as we have said, that God has One Only Son, One naturally begotten; who began not His being when He was born in Bethlehem, but Before All Ages. For hear the Prophet Micah saying, And thou, Bethlehem, house of Ephrata, art little to be among the thousands of Judah. Out of you shall come forth unto Me a Ruler, who shall feed My people Israel: and His goings forth are from the beginning, from days of eternity. Think not then of Him who is now come forth out of Bethlehem, but worship Him who was eternally begotten of the Father.
Suffer none to speak of a beginning of the Son in time, but as a timeless Beginning acknowledge the Father. For the Father is the Beginning of the Son, timeless, incomprehensible, without beginning. The fountain of the river of righteousness, even of the Only-begotten, is the Father, who begot Him as Himself only knows. And would you know that our Lord Jesus Christ is King Eternal? Hear Him again saying, Your father Abraham rejoiced to see My day, and he saw it, and was glad. And then, when the Jews received this hardly, He says what to them was still harder, Before Abraham was, I am. And again He says to the Father, And now, Father, glorify Thou Me with Your own self, with the glory which I had with You before the world was. He says plainly, “before the world was, I had the glory which is with You.”
And again when He says, For You loved Me before the foundation of the world, He plainly declares, “The glory which I have with you is from eternity.”
Source: Catechetical Lectures (New Advent)