4 Thus of what happened to Elias we are ignorant; but still we believe the truthful declarations of Scripture regarding him. Of one thing we are certain, that what God willed happened, and that except by God's will nothing can happen to any one. So, if I am told that it is possible that the flesh of a certain man shall be changed into a celestial body, I allow the possibility, but I cannot tell whether it will be done; and the reason of my ignorance is, that I am not acquainted with the will of God in the matter.
That it will be done if it is God's will, is perfectly clear and indubitable. Again, if I am told that something would happen if God did not prevent it from happening, I reply confidently that what is to happen is the action of God, not the event which might otherwise have happened. For God knows His own future action, and therefore He knows also the effect of that action in preventing the happening of what would otherwise have happened; and, beyond all question, what God knows is more certain than what man thinks.
Hence it is as impossible for what is future not to happen, as for what is past not to have happened; for it can never be God's will that anything should, in the same sense, be both true and false. Therefore all that is properly future cannot but happen; what does not happen never was future; even as all things which are properly in the past did indubitably take place.
Source: Reply to Faustus the Manichaean (New Advent)