Objection 1. It would seem that reviling or railing is not a mortal sin. For no mortal sin is an act of virtue. Now railing is the act of a virtue, viz. of wittiness eutrapelia [Cf. I-II, 60, 5] to which it pertains to rail well, according to the Philosopher (Ethic. iv, 8). Therefore railing or reviling is not a mortal sin.
Objection 2. Further, mortal sin is not to be found in perfect men; and yet these sometimes give utterance to railing or reviling. Thus the Apostle says (Galatians 3:1): "O senseless Galatians!," and our Lord said (Luke 24:25): "O foolish and slow of heart to believe!" Therefore railing or reviling is not a mortal sin.
Objection 3. Further, although that which is a venial sin by reason of its genus may become mortal, that which is mortal by reason of its genus cannot become venial, as stated above (I-II, 88, 4,6). Hence if by reason of its genus it were a mortal sin to give utterance to railing or reviling, it would follow that it is always a mortal sin. But this is apparently untrue, as may be seen in the case of one who utters a reviling word indeliberately or through slight anger. Therefore reviling or railing is not a mortal sin, by reason of its genus.
On the contrary, Nothing but mortal sin deserves the eternal punishment of hell. Now railing or reviling deserves the punishment of hell, according to Matthew 5:22, "Whosoever shall say to his brother... Thou fool, shall be in danger of hell fire." Therefore railing or reviling is a mortal sin.
I answer that, As stated above (Article 1), words are injurious to other persons, not as sounds, but as signs, and this signification depends on the speaker's inward intention. Hence, in sins of word, it seems that we ought to consider with what intention the words are uttered. Since then railing or reviling essentially denotes a dishonoring, if the intention of the utterer is to dishonor the other man, this is properly and essentially to give utterance to railing or reviling: and this is a mortal sin no less than theft or robbery, since a man loves his honor no less than his possessions. If, on the other hand, a man says to another a railing or reviling word, yet with the intention, not of dishonoring him, but rather perhaps of correcting him or with some like purpose, he utters a railing or reviling not formally and essentially, but accidentally and materially, in so far to wit as he says that which might be a railing or reviling. Hence this may be sometimes a venial sin, and sometimes without any sin at all. Nevertheless there is need of discretion in such matters, and one should use such words with moderation, because the railing might be so grave that being uttered inconsiderately it might dishonor the person against whom it is uttered. On such a case a man might commit a mortal sin, even though he did not intend to dishonor the other man: just as were a man incautiously to injure grievously another by striking him in fun, he would not be without blame.
Reply to Objection 1. It belongs to wittiness to utter some slight mockery, not with intent to dishonor or pain the person who is the object of the mockery, but rather with intent to please and amuse: and this may be without sin, if the due circumstances be observed. on the other hand if a man does not shrink from inflicting pain on the object of his witty mockery, so long as he makes others laugh, this is sinful, as stated in the passage quoted.
Reply to Objection 2. Just as it is lawful to strike a person, or damnify him in his belongings for the purpose of correction, so too, for the purpose of correction, may one say a mocking word to a person whom one has to correct. It is thus that our Lord called the disciples "foolish," and the Apostle called the Galatians "senseless." Yet, as Augustine says (De Serm. Dom. in Monte ii, 19), "seldom and only when it is very necessary should we have recourse to invectives, and then so as to urge God's service, not our own."
Reply to Objection 3. Since the sin of railing or reviling depends on the intention of the utterer, it may happen to be a venial sin, if it be a slight railing that does not inflict much dishonor on a man, and be uttered through lightness of heart or some slight anger, without the fixed purpose of dishonoring him, for instance when one intends by such a word to give but little pain.
Source: Summa Theologica (New Advent)