(CG 1/015). That God is Eternal (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

EVERYTHING that has in its substance an admixture of potentiality, to the extent that it has potentiality is liable not to be: because what can be, can also not be. But God in Himself cannot not be, seeing that He is everlasting; therefore there is in God no potentiality.

2. Although in order of time that which is sometimes in potentiality, sometimes in actuality, is in potentiality before it is in actuality, yet, absolutely speaking, actuality is prior to potentiality, because potentiality does not bring itself into actuality, but is brought into actuality by something which is already in actuality. Everything therefore that is any way in potentiality has something else prior to it. But God is the First Being and the First Cause, and therefore has not in Himself any admixture of potentiality.

4. Everything acts inasmuch as it is in actuality. Whatever then is not all actuality, does not act by its whole self, but by something of itself. But what does not act by its whole self, is not a prime agent; for it acts by participation in something else, not by its own essence. The prime agent then, which is God, has no admixture of potentiality, but is pure actuality.

6. We see that there is that in the world which passes from potentiality to actuality. But it does not educe itself from potentiality to actuality, because what is in potentiality is not as yet, and therefore cannot act. Therefore there must be some other prior thing, whereby this thing may be brought out from potentiality to actuality. And again, if this further thing is going out from potentiality to actuality, there must be posited before it yet some other thing, whereby it may be reduced to actuality. But this process cannot go on for ever: therefore we must come to something that is only in actuality, and nowise in potentiality; and that we call God.