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 Cor. 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 Obj. 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 Obj. 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 Obj. 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. _______________________
SECOND ARTICLE [II-II, Q. 42, Art. 2]
Whether Sedition Is Always a Mortal Sin?
Objection 1: It would seem that sedition is not always a mortal sin. For sedition denotes "a tumult tending to fight," according to the gloss quoted above (A. 1). But fighting is not always a mortal sin, indeed it is sometimes just and lawful, as stated above (Q. 40, A. 1). Much more, therefore, can sedition be without a mortal sin.
Obj. 2: Further, sedition is a kind of discord, as stated above (A. 1, ad 3). Now discord can be without mortal sin, and sometimes without any sin at all. Therefore sedition can be also.
Obj. 3: Further, it is praiseworthy to deliver a multitude from a tyrannical rule. Yet this cannot easily be done without some dissension in the multitude, if one part of the multitude seeks to retain the tyrant, while the rest strive to dethrone him. Therefore there can be sedition without mortal sin.
On the contrary, The Apostle forbids seditions together with other things that are mortal sins (2 Cor. 12:20).