(CG 1/014). That in order to a Knowledge of God we must use the Method of... (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

THE beginning of anything and its ceasing to be is brought about by motion or change. But it has been shown that God is altogether unchangeable: He is therefore eternal, without beginning or end.

2. Those things alone are measured by time which are in motion, inasmuch as time is an enumeration of motion. But God is altogether without motion, and therefore is not measured by time. Therefore in Him it is impossible to fix any before or after: He has no being after not being, nor can He have any not being after being, nor can any succession be found in His being, because all this is unintelligible without time. He is therefore without beginning and without end, having all His being at once, wherein consists the essence of eternity.

3. If at some time God was not, and afterwards was, He was brought forth by some cause from not being to being. But not by Himself, because what is not cannot do anything. But if by another, that other is prior to Him. But it has been shown that God is the First Cause; therefore He did not begin to be: hence neither will He cease to be; because what always has been has the force of being always.

4. We see in the world some things which are possible to be and not to be. But everything that is possible to be has a cause: for seeing that of itself it is open to two alternatives, being and not being; if being is to be assigned to it, that must be from some cause. But we cannot proceed to infinity in a series of causes: therefore we must posit something that necessarily is. Now everything necessary either has the cause of its necessity from elsewhere, or not from elsewhere, but is of itself necessary. But we cannot proceed to infinity in the enumeration of things necessary that have the cause of their necessity from elsewhere: therefore we must come to some first thing necessary, that is of itself necessary; and that is God. Therefore God is eternal, since everything that is of itself necessary is eternal.

Hence the Psalmist: But you, O Lord, abidest for ever: you are the self-same, and your years shall not fail (Ps. ci, 13-28).