Objection 1. It would seem that things known or declared prophetically can be false. For prophecy is about future contingencies, as stated above (Article 3). Now future contingencies may possibly not happen; else they would happen of necessity. Therefore the matter of prophecy can be false.
Objection 2. Further, Isaias prophesied to Ezechias saying (Isaiah 38:1): "Take order with thy house, for thou shalt surely die, and shalt not live," and yet fifteen years were added to his life (2 Kings 20:6). Again the Lord said (Jeremiah 18:7-8): "I will suddenly speak against a nation and against a kingdom, to root out and to pull down and to destroy it. If that nation against which I have spoken shall repent of their evil, I also will repent of the evil that I have thought to do them." This is instanced in the example of the Ninevites, according to John 3:10: "The Lord [Vulgate: 'God'] had mercy with regard to the evil which He had said that He would do to them, and He did it not." Therefore the matter of prophecy can be false.
Objection 3. Further, in a conditional proposition, whenever the antecedent is absolutely necessary, the consequent is absolutely necessary, because the consequent of a conditional proposition stands in the same relation to the antecedent, as the conclusion to the premises in a syllogism, and a syllogism whose premises are necessary always leads to a necessary conclusion, as we find proved in I Poster. 6. But if the matter of a prophecy cannot be false, the following conditional proposition must needs be true: "If a thing has been prophesied, it will be." Now the antecedent of this conditional proposition is absolutely necessary, since it is about the past. Therefore the consequent is also necessary absolutely; yet this is unfitting, for then prophecy would not be about contingencies. Therefore it is untrue that the matter of prophecy cannot be false.
On the contrary, Cassiodorus says [Prol. in Psalt. i] that "prophecy is a Divine inspiration or revelation, announcing the issue of things with invariable truth." Now the truth of prophecy would not be invariable, if its matter could be false. Therefore nothing false can come under prophecy.
I answer that, As may be gathered from what has been said (1 and 3 and 5), prophecy is a kind of knowledge impressed under the form of teaching on the prophet's intellect, by Divine revelation. Now the truth of knowledge is the same in disciple and teacher since the knowledge of the disciple is a likeness of the knowledge of the teacher, even as in natural things the form of the thing generated is a likeness of the form of the generator. Jerome speaks in this sense when he says [Comment. in Daniel 2:10 that "prophecy is the seal of the Divine foreknowledge." Consequently the same truth must needs be in prophetic knowledge and utterances, as in the Divine knowledge, under which nothing false can possibly come, as stated in I, 16, 8. Therefore nothing false can come under prophecy.
Reply to Objection 1. As stated in I, 14, 13 the certitude of the Divine foreknowledge does not exclude the contingency of future singular events, because that knowledge regards the future as present and already determinate to one thing. Wherefore prophecy also, which is an "impressed likeness" or "seal of the Divine foreknowledge," does not by its unchangeable truth exclude the contingency of future things.
Reply to Objection 2. The Divine foreknowledge regards future things in two ways. First, as they are in themselves, in so far, to wit, as it sees them in their presentiality: secondly, as in their causes, inasmuch as it sees the order of causes in relation to their effects. And though future contingencies, considered as in themselves, are determinate to one thing, yet, considered as in their causes, they are not so determined but that they can happen otherwise. Again, though this twofold knowledge is always united in the Divine intellect, it is not always united in the prophetic revelation, because an imprint made by an active cause is not always on a par with the virtue of that cause. Hence sometimes the prophetic revelation is an imprinted likeness of the Divine foreknowledge, in so far as the latter regards future contingencies in themselves: and such things happen in the same way as foretold, for example this saying of Isaiah 7:14: "Behold a virgin shall conceive." Sometimes, however, the prophetic revelation is an imprinted likeness of the Divine foreknowledge as knowing the order of causes to effects; and then at times the event is otherwise than foretold. Yet the prophecy does not cover a falsehood, for the meaning of the prophecy is that inferior causes, whether they be natural causes or human acts, are so disposed as to lead to such a result. On this way we are to understand the saying of Isaiah 38:1: "Thou shalt die, and not live"; in other words, "The disposition of thy body has a tendency to death": and the saying of Jonah 3:4, "Yet forty days, and Nineveh shall be destroyed," that is to say, "Its merits demand that it should be destroyed." God is said "to repent," metaphorically, inasmuch as He bears Himself after the manner of one who repents, by "changing His sentence, although He changes not His counsel" [Cf. I, 19, 7, ad 2].
Reply to Objection 3. Since the same truth of prophecy is the same as the truth of Divine foreknowledge, as stated above, the conditional proposition: "If this was prophesied, it will be," is true in the same way as the proposition: "If this was foreknown, it will be": for in both cases it is impossible for the antecedent not to be. Hence the consequent is necessary, considered, not as something future in our regard, but as being present to the Divine foreknowledge, as stated in I, 14, 13, ad 2.
Source: Summa Theologica (New Advent)