Augustine of Hippo

In saying this, he wanted it to be understood from their confession that they had chosen to live in him by not believing in the kingdom of the devil and that the devil could not stand divided against himself. So let the Pharisees choose what they want. If Satan could not cast out Satan, they could find nothing to say against the Lord. But if Satan can cast out Satan, let them look out for themselves all the more and let them abandon his kingdom because it cannot stand divided against itself.

Sermon 71.1 ↗

Jerome

And if Satan casts out Satan, he is divided against himself; how then will his kingdom stand? A kingdom and a city divided against themselves cannot endure; but just as small things grow by harmony, so great things are destroyed by discord. Therefore, if Satan fights against himself, and a demon is an enemy of a demon, then the end of the world should have already come; so that opposing powers would not have a place in it, and their war would be the peace of mankind. …

Commentary on Matthew ↗

Thomas Aquinas

If Satan casts out Satan, he is divided against himself. Casting out implies violent motion; therefore, it is necessary that where there is contradiction there is division: "There are always quarrels between the proud" (Pr 13:10). But someone could say that there was no casting out, because he went out voluntarily. But this has no force, because such a departure is not a casting out, but takes place by obeying someone commanding; hence this would be a voluntary departure. …

Commentary on Matthew ↗