Objection 1. It would seem that contention is not a mortal sin. For there is no mortal sin in spiritual men: and yet contention is to be found in them, according to Luke 22:24: "And there was also a strife amongst" the disciples of Jesus, "which of them should... be the greatest." Therefore contention is not a mortal sin.
Objection 2. Further, no well disposed man should be pleased that his neighbor commit a mortal sin. But the Apostle says (Philippians 1:17): "Some out of contention preach Christ," and afterwards he says (Philippians 1:18): "In this also I rejoice, yea, and will rejoice." Therefore contention is not a mortal sin.
Objection 3. Further, it happens that people contend either in the courts or in disputations, without any spiteful purpose, and with a good intention, as, for example, those who contend by disputing with heretics. Hence a gloss on 1 Samuel 14:1, "It came to pass one day," etc. says: "Catholics do not raise contentions with heretics, unless they are first challenged to dispute." Therefore contention is not a mortal sin.
Objection 4. Further, Job seems to have contended with God, according to Job 39:32: "Shall he that contendeth with God be so easily silenced?" And yet Job was not guilty of mortal sin, since the Lord said of him (Job 42:7): "You have not spoken the thing that is right before me, as my servant Job hath." Therefore contention is not always a mortal sin.
On the contrary, It is against the precept of the Apostle who says (2 Timothy 2:14): "Contend not in words." Moreover (Galatians 5:20) contention is included among the works of the flesh, and as stated there (Galatians 5:21) "they who do such things shall not obtain the kingdom of God." 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. For this reason when a man contrasts various contrary things in a speech, this is called "contentio," which Tully calls one of the rhetorical colors (De Rhet. ad Heren. iv), where he says that "it consists in developing a speech from contrary things," for instance: "Adulation has a pleasant beginning, and a most bitter end."
Now contrariety of speech may be looked at in two ways: first with regard to the intention of the contentious party, secondly, with regard to the manner of contending. As to the intention, we must consider whether he contends against the truth, and then he is to be blamed, or against falsehood, and then he should be praised. As to the manner, we must consider whether his manner of contending is in keeping with the persons and the matter in dispute, for then it would be praiseworthy, hence Tully says (De Rhet. ad Heren. iii) that "contention is a sharp speech suitable for proof and refutation"--or whether it exceeds the demands of the persons and matter in dispute, in which case it is blameworthy.
Accordingly if we take contention as denoting a disclaimer of the truth and an inordinate manner, it is a mortal sin. Thus Ambrose [Cf. Gloss. Ord. in Rom. i, 29 defines contention: "Contention is a disclaimer of the truth with clamorous confidence." If, however, contention denote a disavowal of what is false, with the proper measure of acrimony, it is praiseworthy: whereas, if it denote a disavowal of falsehood, together with an inordinate manner, it can be a venial sin, unless the contention be conducted so inordinately, as to give scandal to others. Hence the Apostle after saying (2 Timothy 2:14): "Contend not in words," adds, "for it is to no profit, but to the subverting of the hearers."
Reply to Objection 1. The disciples of Christ contended together, not with the intention of disclaiming the truth, since each one stood up for what he thought was true. Yet there was inordinateness in their contention, because they contended about a matter which they ought not to have contended about, viz. the primacy of honor; for they were not spiritual men as yet, as a gloss says on the same passage; and for this reason Our Lord checked them.
Reply to Objection 2. Those who preached Christ "out of contention," were to be blamed, because, although they did not gainsay the truth of faith, but preached it, yet they did gainsay the truth, by the fact that they thought they would "raise affliction" to the Apostle who was preaching the truth of faith. Hence the Apostle rejoiced not in their contention, but in the fruit that would result therefrom, namely that Christ would be made known--since evil is sometimes the occasion of good results.
Reply to Objection 3. 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. On 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.
Reply to Objection 4. Contention here denotes an ordinary dispute. For Job had said (13:3): "I will speak to the Almighty, and I desire to reason with God": yet he intended not to impugn the truth, but to defend it, and in seeking the truth thus, he had no wish to be inordinate in mind or in speech.
Source: Summa Theologica (New Advent)