St. Gregory the Theologian, Oration 38, Section 8:
The absence of limit is contemplated in two ways, with regard to the beginning and to the end, for that which is above both and is not contained between them is without limit. When the mind gazes steadfastly into the the depth above, not having a place to stand and relying on the representations it has of God, from this it has of God, from this perspective it names as “without beginning” that which is without limit and without outlet. Yet when it gazes at what is below and what is subsequent, it names it “immortal” and “indestructible”; and when it views the whole together, “eternal.” For eternity is neither time nor some part of time, nor is it measurable, but what is time for us, measured by the movement of the sun, is for everlasting beings eternity, since it is coextensive with these things, as if it were a kind of movement and interval of time.
For me this is enough reflection about God for now. For it is not the time to go beyond these things, since our concern heroes not “theology” but “economy.” When I say “God,” I mean Father and Son and Holy Spirit. The divinity is not diffused beyond these, lest we introduce a crowd of gods, but nor is it limited to fewer than these, lest we be condemned to a poverty of divinity, either Judaizing because of the monarchy or hellenizing because of the abundance. For the evil is alike in both cases, though it is found in opposites. This then is the Holy of Holies, which is veiled by the seraphim and glorified with a threefold “holy,” converging in one lordship and divinity, which another who preceded us has explained in a most beautiful and exalted way.