St. Thomas cites St. Isidore, ‘the reviler is hasty and bursts out in injurious words’; he cites a gloss on Rm 1:30, ‘the contumelious are those who by word or deed revile and shame others.’

St. Thomas says railing and taunts are like reviling as they too expose a man’s faults to the detriment of his honor. He says a man’s faults are exposed in three ways.

First, reviling or railing dishonors a man by exposing his fault through guilt; second, taunts dishonor a man by exposing his fault through guilt and punishment.  “Hence,” St. Thomas writes,  “if one man spitefully says to another, “you are blind!”, he taunts but does not revile him; whereas if he says, “you thief!”, he not only taunts but reviles him.”  Again, reviling directly reveals a moral defect in the soul; taunts associate moral defects to the body.  Third, upbraiding dishonors a man by drawing attention to his inferiority in order to lessen the honor due him for any kind of excellence.  St. Thomas says this properly happens when one spitefully reminds a man that one helped him when he was in need, i.e. “You ingrate! have you forgotten what so and so did for you...”  

St. Thomas says all these terms are sometimes mixed.  

Whether reviling is a mortal sin?

Reviling words wound charity and justice; they are the kinds of words our Savior warned us not to use, “Whosoever shall say to his brother... thou fool! Shall be in danger of hell fire.”

St. Thomas says sinful reviling depends on the speaker’s intention.  If the speaker intends to dishonor the other man, this is a mortal sin... no less than theft or robbery since every man has a right to his possessions.  If, however, the speaker only wishes to correct the man his insult may be only a venial sin or no sin at all.   

But St. Thomas advises caution,
“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.  In 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.”

 Whether one ought to suffer oneself to be reviled?
 
“Just as we need patience in things done against us, so do we need it in things said against us.”

In general, we should be deaf to revilers. “They that sought evils to me spoke vain things... but as a deaf man, I heard not; and as one dumb, I opened not my mouth.”  (Ps 37:13-14)

Silence must not be used as an act of revenge, i.e. so that the reviler becomes angry.  Rather, praiseworthy silence is that which gives place to anger, “Strive not with a man that is full of tongue, and heap not wood upon his fire.” (Ecclus. 8:4)

Nevertheless, sometimes the reviler should be withstood and this for two reasons: first, for the good of the reviler; second, for the good of others who may otherwise be hindered from virtue on account of the one being reviled.

Whether reviling arises from anger?

St. Thomas says reviling chiefly arises from anger.  Reviling is closely connected to anger’s end, which is revenge.  The easiest way for an angry man to take revenge on another man is to revile him.

Now that is a summary on St. Thomas on reviling. 

Perhaps we want to defend our reviling or use of insulting words, “I revile only to correct a man not deprive him of honor; I am following the Lord’s example who reviled evil doers.”  

This may be true, but we must be cautious against claiming to ourselves what is not ours.  Our Lord alone reads souls; He knows absolutely, we know only in part.  We do well to follow our Lord’s example, but we must do so realizing our limitations.  Our Lord also walked on water, but we don’t try this and for good reason. “The reviler is checked with moderation - as a duty of charity, not through lust for one’s own honor. ‘Answer not a fool according to his own folly, lest thou be like him.’” 

St. Thomas says our corrections should be given with moderation; I would add, and divine help.

Months ago, when there seemed a daily priest scandal making news, a priest went into a local postal store to mail a package.  The line was longer than usual and when he got there other patrons quickly lined up behind him including a well dressed man who piped up, “Those Catholic priests, they’re no better than anyone else!  Did you read what father so and so did!...” 

The priest, pretending not to hear, remained silent.  The man continued his insulting banter.  The priest prayed that he would tire or that his listener would change topics.  No such rescue came.

So the priest prayed for guidance; the right words, the right time, the man’s betterment,... then he knew what he must do.

Right before the counter became available, the priest turned and addressed the man,  “Sir, I could not help overhearing you, but there is one big difference between a priest who does those ugly things and anyone else who does them.”  And, pausing to catch all ears, the priest ended, “If a priest does those things, he goes deeper into hell.”

Then the priest turned and began his purchase in a very, very quiet postal store.