Daily Reading for March 9 • Gregory, Bishop of Nyssa, c. 394
One mark of the true Godhead is indicated by the words of Holy Scripture, which Moses learnt by the voice from heaven, when he heard him who said, “I am He that is.” We think it right, then, to believe that to be alone truly Divine which is represented as eternal and infinite in respect of being; and all that is contemplated therein is always the same, neither growing nor being consumed; so that if one should say of God, that formerly he was, but now is not, or that he now is, but formerly was not, we should consider each of the sayings alike to be godless: for by both alike the idea of eternity is mutilated, being cut short on one side or the other by non-existence, whether one contemplates “nothing” as preceding “being,” or declares that “being” ends in “nothing.”. . .
For this reason we declare the maintenance of their doctrine as to the non-existence at some time of him who truly is, to be a denial and rejection of his true Godhead; and this on the ground that . . . he who showed himself to Moses by the light speaks of himself as being, when he says, “I am He that is.”. . .
If then Moses lays down as a law for us some such mark of true Godhead as this, that we know nothing else of God but this one thing, that he is . . . we declare all the sophistical fabrication about the non-existence at some time of him who truly is, to be nothing else than a departure from Christianity, and a turning to idolatry. For when the Evangelist, in his discourse concerning the Nature of God, separates at all points non-existence from him who is, and, by his constant repetition of the word “was,” carefully destroys the suspicion of non-existence, and calls him the Only-begotten God, the Word of God, the Son of God, equal with God, and all such names, we have this judgment fixed and settled in us, that if the Only-begotten Son is God, we must believe that he who is believed to be God is eternal. And indeed he is verily God, and assuredly is eternal, and is never at any time found to be non-existent.
From Against Eunomius by Gregory of Nyssa, Book VIII; found at http://www.ccel.org/ccel/schaff/npnf205.viii.i.x.i.html