Objection 1. It would seem that prophecy is only about future contingencies. For Cassiodorus says [Prol. super Psalt. i] that "prophecy is a Divine inspiration or revelation, announcing the issue of things with unchangeable truth." Now issues pertain to future contingencies. Therefore the prophetic revelation is about future contingencies alone.
Objection 2. Further, according to 1 Corinthians 12, the grace of prophecy is differentiated from wisdom and faith, which are about Divine things; and from the discernment of spirits, which is about created spirits; and from knowledge, which is about human things. Now habits and acts are differentiated by their objects, as stated above (I-II, 54, 2). Therefore it seems that the object of prophecy is not connected with any of the above. Therefore it follows that it is about future contingencies alone.
Objection 3. Further, difference of object causes difference of species, as stated above (I-II, 54, 2). Therefore, if one prophecy is about future contingencies, and another about other things, it would seem to follow that these are different species of prophecy.
On the contrary, Gregory says (Hom. i super Ezech.) that some prophecies are "about the future, for instance (Isaiah 7:14), 'Behold a virgin shall conceive, and bear a son'"; some are "about the past, as (Genesis 1:1), 'In the beginning God created heaven and earth'"; some are "about the present," as (1 Corinthians 14:24-25), "If all prophesy, and there come in one that believeth not... the secrets of his heart are made manifest." Therefore prophecy is not about future contingencies alone.
I answer that, A manifestation made by means of a certain light can extend to all those things that are subject to that light: thus the body's sight extends to all colors, and the soul's natural knowledge extends to whatever is subject to the light of the active intellect. Now prophetic knowledge comes through a Divine light, whereby it is possible to know all things both Divine and human, both spiritual and corporeal; and consequently the prophetic revelation extends to them all. Thus by the ministry of spirits a prophetic revelation concerning the perfections of God and the angels was made to Isaiah 6:1, where it is written, "I saw the Lord sitting upon a throne high and elevated." Moreover his prophecy contains matters referring to natural bodies, according to the words of Isaiah 40:12, "Who hath measured the waters in the hollow of His hand," etc. It also contains matters relating to human conduct, according to Isaiah 58:1, "Deal thy bread to the hungry," etc.; and besides this it contains things pertaining to future events, according to Isaiah 47:9, "Two things shall come upon thee suddenly in one day, barrenness and widowhood."
Since, however, prophecy is about things remote from our knowledge, it must be observed that the more remote things are from our knowledge the more pertinent they are to prophecy. Of such things there are three degrees. One degree comprises things remote from the knowledge, either sensitive or intellective, of some particular man, but not from the knowledge of all men; thus a particular man knows by sense things present to him locally, which another man does not know by human sense, since they are removed from him. Thus Eliseus knew prophetically what his disciple Giezi had done in his absence (2 Kings 5:26), and in like manner the secret thoughts of one man are manifested prophetically to another, according to 1 Corinthians 14:25; and again in this way what one man knows by demonstration may be revealed to another prophetically.
The second degree comprises those things which surpass the knowledge of all men without exception, not that they are in themselves unknowable, but on account of a defect in human knowledge; such as the mystery of the Trinity, which was revealed by the Seraphim saying: "Holy, Holy, Holy," etc. (Isaiah 6:3).
The last degree comprises things remote from the knowledge of all men, through being in themselves unknowable; such are future contingencies, the truth of which is indeterminate. And since that which is predicated universally and by its very nature, takes precedence of that which is predicated in a limited and relative sense, it follows that revelation of future events belongs most properly to prophecy, and from this prophecy apparently takes its name. Hence Gregory says (Hom. i super Ezech.): "And since a prophet is so called because he foretells the future, his name loses its significance when he speaks of the past or present."
Reply to Objection 1. Prophecy is there defined according to its proper signification; and it is in this sense that it is differentiated from the other gratuitous graces.
Reply to Objection 2. This is evident from what has just been said. We might also reply that all those things that are the matter of prophecy have the common aspect of being unknowable to man except by Divine revelation; whereas those that are the matter of "wisdom," "knowledge," and the "interpretation of speeches," can be known by man through natural reason, but are manifested in a higher way through the enlightening of the Divine light. As to "faith," although it is about things invisible to man, it is not concerned with the knowledge of the things believed, but with a man's certitude of assent to things known by others.
Reply to Objection 3. The formal element in prophetic knowledge is the Divine light, which being one, gives unity of species to prophecy, although the things prophetically manifested by the Divine light are diverse.
Source: Summa Theologica (New Advent)