Thomas Aquinas

“ Contention is complete and is a mortal sin when, in contending before a judge, a man gainsays the truth of justice, or in a disputation, intends to impugn the true doctrine. In this sense Catholics do not contend against heretics, but the reverse. But when, whether in court or in a disputation, it is incomplete, i.e. in respect of the acrimony of speech, it is not always a mortal sin. ”

“ Now it is proper to pride and vainglory to seek one's own glory. And just as people are discordant when they hold to their own opinion in their hearts, so are they contentious when each defends his own opinion by words. Consequently contention is reckoned a daughter of vainglory for the same reason as discord. ”

“ Now whatever excludes a man from the kingdom of God and is against a precept, is a mortal sin. Therefore contention is a mortal sin.
'''''''I answer that,''''''' To contend is to tend against some one. Wherefore just as discord denotes a contrariety of wills, so contention signifies contrariety of speech. ”