Objection 1. It would seem that sedition is not a special sin distinct from other sins. For, according to Isidore (Etym. x), "a seditious man is one who sows dissent among minds, and begets discord." Now, by provoking the commission of a sin, a man sins by no other kind of sin than that which he provoked. Therefore it seems that sedition is not a special sin distinct from discord.
Objection 2. Further, sedition denotes a kind of division. Now schism takes its name from scission, as stated above (Question 39, Article 1). Therefore, seemingly, the sin of sedition is not distinct from that of schism.
Objection 3. Further, every special sin that is distinct from other sins, is either a capital vice, or arises from some capital vice. Now sedition is reckoned neither among the capital vices, nor among those vices which arise from them, as appears from Moral. xxxi, 45, where both kinds of vice are enumerated. Therefore sedition is not a special sin, distinct from other sins.
On the contrary, Seditions are mentioned as distinct from other sins (2 Corinthians 12:20).
I answer that, Sedition is a special sin, having something in common with war and strife, and differing somewhat from them. It has something in common with them, in so far as it implies a certain antagonism, and it differs from them in two points. First, because war and strife denote actual aggression on either side, whereas sedition may be said to denote either actual aggression, or the preparation for such aggression. Hence a gloss on 2 Corinthians 12:20 says that "seditions are tumults tending to fight," when, to wit, a number of people make preparations with the intention of fighting. Secondly, they differ in that war is, properly speaking, carried on against external foes, being as it were between one people and another, whereas strife is between one individual and another, or between few people on one side and few on the other side, while sedition, in its proper sense, is between mutually dissentient parts of one people, as when one part of the state rises in tumult against another part. Wherefore, since sedition is opposed to a special kind of good, namely the unity and peace of a people, it is a special kind of sin.
Reply to Objection 1. A seditious man is one who incites others to sedition, and since sedition denotes a kind of discord, it follows that a seditious man is one who creates discord, not of any kind, but between the parts of a multitude. And the sin of sedition is not only in him who sows discord, but also in those who dissent from one another inordinately.
Reply to Objection 2. Sedition differs from schism in two respects. First, because schism is opposed to the spiritual unity of the multitude, viz. ecclesiastical unity, whereas sedition is contrary to the temporal or secular unity of the multitude, for instance of a city or kingdom. Secondly, schism does not imply any preparation for a material fight as sedition does, but only for a spiritual dissent.
Reply to Objection 3. Sedition, like schism, is contained under discord, since each is a kind of discord, not between individuals, but between the parts of a multitude.
Source: Summa Theologica (New Advent)