Summa contra Gentiles1.16 Chapter XVI. That in God there is no passive potentiality

Now if God is eternal, it follows of necessity that He is not in potentiality.

For everything in whose substance there is an admixture of potentiality, is possibly non-existent as regards whatever it has of potentiality, for that which may possibly be may possibly not be. Now God in Himself cannot not be, since He is eternal. Therefore in God there is no potentiality to be.

Again. Although that which is sometimes potential and sometimes actual, is in point of time potential before being actual, nevertheless actuality is simply before potentiality: because potentiality does not bring itself into actuality, but needs to be brought into actuality by something actual. Therefore whatever is in any way potential has something previous to it. Now God is the first being and the first cause, as stated above. Therefore in Him there is no admixture of potentiality.

Again. That which of itself must necessarily be, can nowise be possibly, since what of itself must be necessarily, has no cause, whereas whatever can be possibly, has a cause, as proved above. Now God, in Himself, must necessarily be. Therefore nowise can He be possibly. Therefore no potentiality is to be found in His essence.

Again. Everything acts according as it is actual. Wherefore that which is not wholly actual acts, not by its whole self, but by part of itself. Now that which does not act by its whole self is not the first agent, since it acts by participation of something and not by its essence. Therefore the first agent, which is God, has no admixture of potentiality, but is pure act.

Moreover. Just as it is natural that a thing should act in so far as it is actual, so is it natural for it to be passive in so far as it is in potentiality, for movement is the act of that which is in potentiality. Now God is altogether impassible and immovable, as stated above. Therefore in Him there is no potentiality, namely that which is passive.

Further. We notice in the world something that passes from potentiality to actuality. Now it does not reduce itself from potentiality to actuality, because that which is potential is not yet, wherefore neither can it act. Therefore it must be preceded by something else whereby it can be brought from potentiality to actuality. And if this again passes from potentiality to actuality, it must be preceded by something else, whereby it can be brought from potentiality to actuality. But we cannot go on thus to infinity. Therefore we must come to something that is wholly actual and nowise potential. And this we call God.