Ambrose of Milan

You cannot, then, heretic [Arian], build up a false doctrine on the basis of an analogy of human procreation. Nor can you gather the means for such a purpose from our discussion, for we cannot embrace the greatness of infinite godhead, "of whose greatness there is no end," in our limited speech. If you should seek to give an account of a human's birth, you must certainly point to a time. But the divine generation is above all things. It reaches far and wide, and it rises high above all thought and feeling. For it is written, "No one comes to the Father, except by me." Whatever, therefore, you conceive concerning the Father—yes, even his eternity—you cannot conceive anything concerning him except with the Son's aid nor can any understanding ascend to the Father except through the Son. "This is my dearly beloved Son," the Father said. "Is," please note, means that who he is, and what he is, is [true] forever. Hence David is also moved to say, "O Lord, your Word abides forever in heaven," for what abides fails neither in time nor in eternity.