10 And in very truth, Brethren, I am not likely to discover any temporal resemblances which I can compare to eternity. But as to those which you have discovered, what are they? What have you discovered? That a father is greater in time than his son; and therefore you would have the Son of God to be less in time than the Eternal Father, because you have found that a son is less than a father born in time. Find me an eternal father here, and you have found a resemblance.
Thou findest a son less than a father in time, a temporal son less than a temporal father. Have you found me a temporal son younger than eternal father? Seeing then that in Eternity is stability, but in time variety; in Eternity all things stand still, in time one thing comes, another succeeds; you can find a son of lesser age succeeding his father in the variety of time, for that he himself succeeded to his father also, not a son born in time to a father eternal. How then, Brethren, can we find in the creature anything coeternal, when in the creature we find nothing eternal?
You find an eternal father in the creature, and I will find a coeternal son. But if you find not an eternal father, and the one surpasses the other in you; it is sufficient, that for a resemblance I find something coeval. For what is coeternal is one thing what is coeval another. Every day we call them coeval who have the same measure of times; the one is not preceded by the other in time, yet they both whom we call coeval once began to “be.” Now if I shall be able to discover something which is born coeval with that of which it is born; if two coeval things can be discovered, that which begets, and that which is begotten; we discover in this case things coeval, let us understand in the other things coeternal.
If here I shall find that a thing begotten has begun to be ever since that which begets began to be, we may understand at least that the Son of God did not begin to be, ever since He that begot Him did not begin to be. Lo, brethren, perhaps we may discover something in the creature, which is born of something else, and which yet began to be at the same time as that of which it is born began to be. In the latter case, the one began to be when the other began to be; in the former the one did not begin to be, ever since the other began not to be. The first then is coeval, the second coeternal.
Source: Sermons on Selected Lessons of the New Testament (New Advent)