8 He was begotten before all times, before all worlds. “Begotten before.” Before what, He in Whom is no before? Do not in the least imagine any time before that Nativity of Christ whereby He was begotten of the Father; of that Nativity I am speaking by which He is Son of God Almighty, His Only Son our Lord; of that am I first speaking. Do not imagine in this Nativity a beginning of time; do not imagine any space of eternity in which the Father was and the Son was not.
Since when the Father was, since then the Son. And what is that “since,” where is no beginning? Therefore ever Father without beginning, ever Son without beginning. And how, you will say, was He begotten, if He have no beginning? Of eternal, coeternal. At no time was the Father, and the Son not, and yet Son of Father was begotten. Whence is any manner of similitude to be had? We are among things of earth, we are in the visible creature. Let the earth give me a similitude: it gives none.
Let the element of the waters give me some similitude: it has not whereof to give. Some animal give me a similitude: neither can this do it. An animal indeed engenders, both what engenders and what is engendered: but first is the father, and then is born the son. Let us find the coeval and imagine it coeternal. If we shall be able to find a father coeval with his son, and son coeval with his father, let us believe God the Father coeval with His Son, and God the Son coeternal with His Father.
On earth we can find some coeval, we cannot find any coeternal. Let us stretch the coeval and imagine it coeternal. Some one, it may be, will put you on the stretch, by saying, “When is it possible for a father to be found coeval with his son, or son coeval with his father? That the father may beget he goes before in age; that the son may be begotten, he comes after in age: but this father coeval with son, or son with father, how can it be?” Imagine to yourselves fire as father, its shining as son; see, we have found the coevals.
From the instant that the fire begins to be, that instant it begets the shining: neither fire before shining, nor shining after fire. And if we ask, which begets which? The fire the shining, or the shining the fire? Immediately ye conceive by natural sense, by the innate wit of your minds ye all cry out, The fire the shining, not the shining the fire. Lo, here you have a father beginning; lo, a son at the same time, neither going before nor coming after. Lo, here then is a father beginning, lo, a son at the same time beginning.
If I have shown you a father beginning, and a son at the same time beginning, believe the Father not beginning, and with Him the Son not beginning either; the one eternal, the other coeternal. If you get on with your learning, you understand: take pains to get on. The being born, you have; but also the growing, you ought to have; because no man begins with being perfect. As for the Son of God, indeed, He could be born perfect, because He was begotten without time, coeternal with the Father, long before all things, not in age, but in eternity.
He then was begotten coeternal, of which generation the Prophet said, “His generation who shall declare?” begotten of the Father without time, He was born of the Virgin in the fullness of times. This nativity had times going before it. In opportunity of time, when He would, when He knew, then was He born: for He was not born without His will. None of us is born because he will, and none of us dies when he will: He, when He would, was born; when He would, He died: how He would, He was born of a Virgin: how He would, He died; on the cross. Whatever He would, He did: because He was in such wise Man that, unseen, He was God; God assuming, Man assumed; One Christ, God and Man.
Source: On the Creed: A Sermon to Catechumens (New Advent)