4 Afterwards, that the unbelievers and ungodly, the enemies of the Christian name, might not suppose by reason of the various heresies and schisms of those who under the Christian name gather together flocks of lost sheep, that the kingdom of Christ also is divided against itself, He next adds, “He that is not with Me is against Me, and he that gathers not with Me, scatters abroad.” He does not say, he who is under the outward profession of My Name; or the form of My Sacrament; but “he who is not with Me is against Me.”
Nor does He say, he who gathers not under the outward profession of My Name; but “he who gathers not with Me, scatters abroad.” Christ's kingdom then is not divided against itself; but men try to divide that which was bought with the price of the Blood of Christ. “For the Lord knows them that are His. And, let every one that names the Name of Christ depart from iniquity.” For if he depart not from iniquity, he belongs not to the kingdom of Christ, even though he name the Name of Christ.
To give then some illustrations for example's sake, the spirit of covetousness, and the spirit of luxuriousness, because the one heaps together, and the other lavishes, are divided against themselves; yet they belong both to the kingdom of the devil. Among idolaters the spirit of Juno and the spirit of Hercules, are divided against themselves; and both belong to the kingdom of the devil. The heathen Christ's enemy, and the Jew Christ's enemy, are divided against themselves; and both belong to the kingdom of the devil.
Arianus and Photinianus both are heretics, and both are divided against themselves. The Donatist and Maximianist both are heretics, and both divided against themselves. All men's vices and errors that are contrary to each other are divided against themselves, and all belong to the kingdom of the devil; therefore his kingdom shall not stand. But the righteous and the ungodly, the believer and the unbeliever, the Catholic and the heretic, are indeed divided against themselves, but they do not belong all to the kingdom of Christ. “The Lord knows them that are His.” Let no one flatter himself upon a mere name. If he would that the Name of the Lord should profit him, let “him that calls upon the Name of the Lord depart from iniquity.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)