Calvin's Commentaries, Vol. 45: Catholic Epistles, tr. by John King, [1847-50], at sacred-texts.com
Though there was a dispute among the ancients respecting this Epistle, yet as the reading of it is useful, and as it contains nothing inconsistent with the purity of apostolic doctrine, and was received as authentic formerly, by some of the best, I willingly add it to the others. Its brevity, moreover, does not require a long statement of its contents; and almost the whole of it is nearly the same with the second chapter of the last Epistle.
As unprincipled men, under the name of Christians, had crept in, whose chief object was to lead the unstable and weak to a profane contempt of God, Jude first shews, that the faithful ought not to have been moved by agents of this kind, by which the Church has always been assailed; and yet he exhorts them carefully to beware of such pests. And to render them more hateful and detestable, he denounces on them the approaching vengeance of God, such as their impiety deserved. Now, if we consider what Satan has attempted in our age, from the commencement of the revived gospel, and what arts he still busily employs to subvert the faith, and the fear of God, what was a useful warning in the time of Jude, is more than necessary in our age. But this will appear more fully as we proceed in reading the Epistle.