Thomas Aquinas

“ 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. ”

“ But sedition runs counter to the common good of the multitude, so that it is always a mortal sin.
Reply to Objection 2: Discord from what is not evidently good, may be without sin, but discord from what is evidently good, cannot be without sin: and sedition is discord of this kind, for it is contrary to the unity of the multitude, which is a manifest good. ”

“ 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. ”