5 Accordingly, to say, if God is almighty, let Him make what has been done to be undone, is in fact to say, if God is almighty, let Him make a thing to be in the same sense both true and false. God can put an end to the existence of anything, when the thing to be put an end to has a present existence; as when He puts an end by death to the existence of any one who has been brought into existence in birth; for in this case there is an actual existence which may be put a stop to.
But when a thing does not exist, the existence cannot be put a stop to. Now, what is past no longer exists and whatever has an existence which can be put an end to cannot be past. What is truly past is no longer present; and the truth of its past existence is in our judgment, not in the thing itself which no longer exists. The proposition asserting anything to be past is true when the thing no longer exists. God cannot make such a proposition false, because He cannot contradict the truth.
The truth in this case, or the true judgment, is first of all in our own mind, when we know and give expression to it. But should it disappear from our minds by our forgetting it, it would still remain as truth. It will always be true that the past thing which is no longer present had an existence; and the truth of its past existence after it has stopped is the same as the truth of its future existence before it began to be. This truth cannot be contradicted by God, in whom abides the supreme and unchangeable truth, and whose illumination is the source of all the truth to be found in any mind or understanding.
Now God is not omnipotent in the sense of being able to die; nor does this inability prevent His being omnipotent. True omnipotence belongs to Him who truly exists, and who alone is the source of all existence, both spiritual and corporeal. The Creator makes what use He pleases of all His creatures; and His pleasure is in harmony with true and unchangeable justice, by which, as by His own nature, He, Himself unchangeable, brings to pass the changes of all changeable things according to the desert of their natures or of their actions.
No one, therefore, would be so foolish as to deny that Elias being a creature of God could be changed either for the worse or for the better; or that by the will of the omnipotent God he could be changed in a manner unusual among men. So we can have no reason for doubting what on the high authority of Scripture is related of him, unless we limit the power of God to things which we are familiar with.
Source: Reply to Faustus the Manichaean (New Advent)