Jude 1:5

I will therefore put you in remembrance, though ye once know this,.... The Alexandrian copy, and some others, and the Vulgate Latin version, read, "knew all things"; but rather it is to be restrained by the following instance of, God's vengeance on unbelievers; which with others is produced, to vindicate the divine conduct in the condemnation of the above persons, and to show that that is certain, and may be expected, since God has always dealt thus with such persons; and this they knew by reading of the Scriptures; at least they had known it once, though it might now be forgotten by them; and they had known it once for all; they had been perfectly acquainted with it; which is said, lest the apostle should be thought to write to persons ignorant, and rude in knowledge, and to show that he wrote nothing new and unheard of, and so should have the more weight and influence upon them; and he thought fit to remind them of it, though they had known it: it is one part of the work of the ministers of the word to put people in mind of what they have known; which is necessary, because of the inattentiveness of hearers, their forgetfulness, and loss of knowledge, and the weakness of some capacities to take in, and retain things; and if the judgment is not more informed hereby, yet the affections may be afresh raised, and grace be drawn out into exercise, and the mind be established and confirmed.