Objection 1. It would seem that a natural disposition is requisite for prophecy. For prophecy is received by the prophet according to the disposition of the recipient, since a gloss of Jerome on Amos 1:2, "The Lord will roar from Sion," says: "Anyone who wishes to make a comparison naturally turns to those things of which he has experience, and among which his life is spent. For example, sailors compare their enemies to the winds, and their losses to a shipwreck. On like manner Amos, who was a shepherd, likens the fear of God to that which is inspired by the lion's roar." Now that which is received by a thing according to the mode of the recipient requires a natural disposition. Therefore prophecy requires a natural disposition.
Objection 2. Further, the considerations of prophecy are more lofty than those of acquired science. Now natural indisposition hinders the considerations of acquired science, since many are prevented by natural indisposition from succeeding to grasp the speculations of science. Much more therefore is a natural disposition requisite for the contemplation of prophecy.
Objection 3. Further, natural indisposition is a much greater obstacle than an accidental impediment. Now the considerations of prophecy are hindered by an accidental occurrence. For Jerome says in his commentary on Matthew [The quotation is from Origen, Hom. vi in Num.] that "at the time of the marriage act, the presence of the Holy Ghost will not be vouchsafed, even though it be a prophet that fulfils the duty of procreation." Much more therefore does a natural indisposition hinder prophecy; and thus it would seem that a good natural disposition is requisite for prophecy.
On the contrary, Gregory says in a homily for Pentecost (xxx in Ev.): "He," namely the Holy Ghost, "fills the boy harpist and makes him a Psalmist; He fills the herdsman plucking wild figs, and makes him a prophet." Therefore prophecy requires no previous disposition, but depends on the will alone of the Holy Ghost, of Whom it is written (1 Corinthians 12:2): "All these things, one and the same Spirit worketh, dividing to every one according as He will."
I answer that, As stated above (Article 1), prophecy in its true and exact sense comes from Divine inspiration; while that which comes from a natural cause is not called prophecy except in a relative sense. Now we must observe that as God Who is the universal efficient cause requires neither previous matter nor previous disposition of matter in His corporeal effects, for He is able at the same instant to bring into being matter and disposition and form, so neither does He require a previous disposition in His spiritual effects, but is able to produce both the spiritual effect and at the same time the fitting disposition as requisite according to the order of nature. More than this, He is able at the same time, by creation, to produce the subject, so as to dispose a soul for prophecy and give it the prophetic grace, at the very instant of its creation.
Reply to Objection 1. It matters not to prophecy by what comparisons the thing prophesied is expressed; and so the Divine operation makes no change in a prophet in this respect. Yet if there be anything in him incompatible with prophecy, it is removed by the Divine power.
Reply to Objection 2. The considerations of science proceed from a natural cause, and nature cannot work without a previous disposition in matter. This cannot be said of God Who is the cause of prophecy.
Reply to Objection 3. A natural indisposition, if not removed, might be an obstacle to prophetic revelation, for instance if a man were altogether deprived of the natural senses. On the same way a man might be hindered from the act of prophesying by some very strong passion, whether of anger, or of concupiscence as in coition, or by any other passion. But such a natural indisposition as this is removed by the Divine power, which is the cause of prophecy.
Source: Summa Theologica (New Advent)