That is properly termed everlasting or eternal which neither had a beginning of existence, nor can ever cease to be what it is. And this is the idea conveyed by John when he says that God is light. Now His wisdom is the splendour of that light, not only in respect of its being light, but also of being everlasting light, so that His wisdom is eternal and everlasting splendour. If this be fully understood, it clearly shows that the existence of the Son is derived from the Father but not in time, nor from any other beginning, except, as we have said, from God Himself.

On First Principles, Bk. 1, ch. 2; par. 11