Objection 1. It seems that God can make the past not to have been. For what is impossible in itself is much more impossible than that which is only impossible accidentally. But God can do what is impossible in itself, as to give sight to the blind, or to raise the dead. Therefore, and much more can He do what is only impossible accidentally. Now for the past not to have been is impossible accidentally: thus for Socrates not to be running is accidentally impossible, from the fact that his running is a thing of the past. Therefore God can make the past not to have been.
Objection 2. Further, what God could do, He can do now, since His power is not lessened. But God could have effected, before Socrates ran, that he should not run. Therefore, when he has run, God could effect that he did not run.
Objection 3. Further, charity is a more excellent virtue than virginity. But God can supply charity that is lost; therefore also lost virginity. Therefore He can so effect that what was corrupt should not have been corrupt.
On the contrary, Jerome says (Ep. 22 ad Eustoch.): "Although God can do all things, He cannot make a thing that is corrupt not to have been corrupted." Therefore, for the same reason, He cannot effect that anything else which is past should not have been.
I answer that, As was said above (Question 7, Article 2), there does not fall under the scope of God's omnipotence anything that implies a contradiction. Now that the past should not have been implies a contradiction. For as it implies a contradiction to say that Socrates is sitting, and is not sitting, so does it to say that he sat, and did not sit. But to say that he did sit is to say that it happened in the past. To say that he did not sit, is to say that it did not happen. Whence, that the past should not have been, does not come under the scope of divine power. This is what Augustine means when he says (Contra Faust. xxix, 5): "Whosoever says, If God is almighty, let Him make what is done as if it were not done, does not see that this is to say: If God is almighty let Him effect that what is true, by the very fact that it is true, be false": and the Philosopher says (Ethic. vi, 2): "Of this one thing alone is God deprived--namely, to make undone the things that have been done."
Reply to Objection 1. Although it is impossible accidentally for the past not to have been, if one considers the past thing itself, as, for instance, the running of Socrates; nevertheless, if the past thing is considered as past, that it should not have been is impossible, not only in itself, but absolutely since it implies a contradiction. Thus, it is more impossible than the raising of the dead; in which there is nothing contradictory, because this is reckoned impossible in reference to some power, that is to say, some natural power; for such impossible things do come beneath the scope of divine power.
Reply to Objection 2. As God, in accordance with the perfection of the divine power, can do all things, and yet some things are not subject to His power, because they fall short of being possible; so, also, if we regard the immutability of the divine power, whatever God could do, He can do now. Some things, however, at one time were in the nature of possibility, whilst they were yet to be done, which now fall short of the nature of possibility, when they have been done. So is God said not to be able to do them, because they themselves cannot be done.
Reply to Objection 3. God can remove all corruption of the mind and body from a woman who has fallen; but the fact that she had been corrupt cannot be removed from her; as also is it impossible that the fact of having sinned or having lost charity thereby can be removed from the sinner.
Source: Summa Theologica (New Advent)