Numbers 16:2

Rose up. The crime of these men, which was punished in so remarkable a manner, was that of schism, and of rebellion against the authority established by God in the Church; and their pretending to the priesthood without being lawfully called and sent: the same is the case of all modern sectaries. (Challoner)

Let them dread a similar punishment; not only the authors of such wicked pretensions, but those also who consent to them, Romans i. 32. For we find that Core and all his adherents were buried in hell; (ver. 33,) and those likewise who complained that their punishment was too severe, fell victims to the raging fire, ver. 49. With what earnestness ought we not, therefore, to contend for the faith once delivered to the saints! (Jude 3.) For if those be so severely punished who rise up in opposition to lawful superiors, either in church or state, what swift destruction do they not bring upon their own heads who deny God, who bought them, and make him a liar, by calling in question his most sacred truths? (2 Peter ii. 1.) (Haydock)

Core and his companions impugned not the law directly, but resisted Moses and Aaron. (St. Ignatius of Antioch, ep. ad Magnes.) They believed in the same God; yet, because they took upon themselves to sacrifice, they were forthwith punished by God, and their unlawful sacrifices could do them no service. (St. Cyprian, ep. i. 6.) Thus we are warned to keep in the true Church, and to obey those who are set over us; and never, for any temporal consideration whatever, to encourage, by our presence, the sermons or meetings of heretics, or of schismatics, lest we perish with them, ver. 26. (Worthington)