Objection 1. It would seem that a good life is requisite for prophecy. For it is written (Wisdom 7:27) that the wisdom of God "through nations conveyeth herself into holy souls," and "maketh the friends of God, and prophets." Now there can be no holiness without a good life and sanctifying grace. Therefore prophecy cannot be without a good life and sanctifying grace.
Objection 2. Further, secrets are not revealed save to a friend, according to John 15:15, "But I have called you friends, because all things whatsoever I have heard of My Father, I have made known to you." Now God reveals His secrets to the prophets (Amos 3:7). Therefore it would seem that the prophets are the friends of God; which is impossible without charity. Therefore seemingly prophecy cannot be without charity; and charity is impossible without sanctifying grace.
Objection 3. Further, it is written (Matthew 7:15): "Beware of false prophets, who come to you in the clothing of sheep, but inwardly they are ravening wolves." Now all who are without grace are likened inwardly to a ravening wolf, and consequently all such are false prophets. Therefore no man is a true prophet except he be good by grace.
Objection 4. Further, the Philosopher says (De Somn. et Vigil. [Cf. De Divinat. per Somn. i, which is annexed to the work quoted]) that "if interpretation of dreams is from God, it is unfitting for it to be bestowed on any but the best." Now it is evident that the gift of prophecy is from God. Therefore the gift of prophecy is vouchsafed only to the best men.
On the contrary, To those who had said, "Lord, have we not prophesied in Thy name?" this reply is made: "I never knew you" (Matthew 7:22-23). Now "the Lord knoweth who are His" (2 Timothy 2:19). Therefore prophecy can be in those who are not God's by grace.
I answer that, A good life may be considered from two points of view. First, with regard to its inward root, which is sanctifying grace. Secondly, with regard to the inward passions of the soul and the outward actions. Now sanctifying grace is given chiefly in order that man's soul may be united to God by charity. Wherefore Augustine says (De Trin. xv, 18): "A man is not transferred from the left side to the right, unless he receive the Holy Ghost, by Whom he is made a lover of God and of his neighbor." Hence whatever can be without charity can be without sanctifying grace, and consequently without goodness of life. Now prophecy can be without charity; and this is clear on two counts. First, on account of their respective acts: for prophecy pertains to the intellect, whose act precedes the act of the will, which power is perfected by charity. For this reason the Apostle (1 Corinthians 13) reckons prophecy with other things pertinent to the intellect, that can be had without charity. Secondly, on account of their respective ends. For prophecy like other gratuitous graces is given for the good of the Church, according to 1 Corinthians 12:7, "The manifestation of the Spirit is given to every man unto profit"; and is not directly intended to unite man's affections to God, which is the purpose of charity. Therefore prophecy can be without a good life, as regards the first root of this goodness.
If, however, we consider a good life, with regard to the passions of the soul, and external actions, from this point of view an evil life is an obstacle to prophecy. For prophecy requires the mind to be raised very high in order to contemplate spiritual things, and this is hindered by strong passions, and the inordinate pursuit of external things. Hence we read of the sons of the prophets (2 Kings 4:38) that they "dwelt together with [Vulgate: 'before']" Eliseus, leading a solitary life, as it were, lest worldly employment should be a hindrance to the gift of prophecy.
Reply to Objection 1. Sometimes the gift of prophecy is given to a man both for the good of others, and in order to enlighten his own mind; and such are those whom Divine wisdom, "conveying itself" by sanctifying grace to their minds, "maketh the friends of God, and prophets." Others, however, receive the gift of prophecy merely for the good of others. Hence Jerome commenting on Matthew 7:22, says: "Sometimes prophesying, the working of miracles, and the casting out of demons are accorded not to the merit of those who do these things, but either to the invoking the name of Christ, or to the condemnation of those who invoke, and for the good of those who see and hear."
Reply to Objection 2. Gregory [Hom. xxvii in Ev.] expounding this passage [John 15:15 says: "Since we love the lofty things of heaven as soon as we hear them, we know them as soon as we love them, for to love is to know. Accordingly He had made all things known to them, because having renounced earthly desires they were kindled by the torches of perfect love." On this way the Divine secrets are not always revealed to prophets.
Reply to Objection 3. Not all wicked men are ravening wolves, but only those whose purpose is to injure others. For Chrysostom says [Opus Imperf. in Matth., Hom. xix, among the works of St. John Chrysostom, and falsely ascribed to him] that "Catholic teachers, though they be sinners, are called slaves of the flesh, but never ravening wolves, because they do not purpose the destruction of Christians." And since prophecy is directed to the good of others, it is manifest that such are false prophets, because they are not sent for this purpose by God.
Reply to Objection 4. God's gifts are not always bestowed on those who are simply the best, but sometimes are vouchsafed to those who are best as regards the receiving of this or that gift. Accordingly God grants the gift of prophecy to those whom He judges best to give it to.
Source: Summa Theologica (New Advent)