Summa contra Gentiles1.15 Chapter XV. That God is eternal

From the foregoing it is also clear that God is eternal.

For whatever begins or ceases to be, suffers this through movement or change. Now it has been shown that God is altogether unchangeable. Therefore He is eternal, having neither beginning nor end.

Again. Only things which are moved are measured by time: because time is the measure of movement, as stated in 4 Phys. Now God is absolutely without movement, as we have already proved. Therefore we cannot mark before and after in Him. Therefore in Him there is not being after non-being, nor can He have non-being after being, nor is it possible to find any succession in His being, because these things cannot be understood apart from time. Therefore He is without beginning and end, and has all His being simultaneously: and in this consists the notion of eternity.

Moreover. If anywhen He was not and afterwards was, He was brought by someone out of non-being into being. Not by Himself; because what is not cannot do anything. And if by another, this other is prior to Him. Now it has been shown that God is the first cause. Therefore He did not begin to be. Therefore neither will He cease to be: because that which always was, has the power to be always. Therefore He is eternal.

Furthermore. We observe that in the world there are certain things which can be and not be, namely those that are subject to generation and corruption. Now whatsoever is possible to be has a cause, because, as in itself it is equally related to two things, namely being and not being, it follows that if it acquires being this is the result of some cause. But, as proved above by Aristotle's argument, we cannot go on to infinity in causes. Therefore we must suppose some thing, which it is necessary to be. Now every necessary thing either has a cause of its necessity from without, or has no such cause, but is necessary of itself. But we cannot go on to infinity in necessary things that have causes of their necessity from without. Therefore we must suppose some first necessary thing which is necessary of itself: and this is God, since He is the first cause, as proved above. Therefore God is eternal, since whatever is necessary of itself is eternal.

Again. Aristotle proves the everlastingness of movement from the everlastingness of time: and thence he goes on to prove the everlastingness of the substance that is the cause of movement. Now the first moving substance is God. Therefore He is everlasting. And supposing the everlastingness of time and movement to be denied, there still remains the argument in proof of the everlastingness of substance. For if movement had a beginning, it must have had its beginning from some mover. And if this mover had a beginning, it had its beginning from some agent. And thus either we shall go on to infinity, or we shall come to something without a beginning.

Divine authority bears witness to this truth: wherefore the Psalm reads: But Thou, O Lord, endurest for ever, and again: But Thou art always the self-same, and Thy years shall not fail.