Though ye once: this may be joined either with the verb following, knew, according to our translation, and the sense is, though ye knew this certainly, as the word once is taken, Psalms 89:35, or perfectly and thoroughly, or once for all; or rather, with what goes before, and the words may be read, I will yet once (viz. while I am in this tabernacle) put you in remembrance of this, though you know it; as 2 Peter 1:12. Having saved the people; the people of Israel. Afterward destroyed them; viz. in the wilderness, by plague, fiery serpents, &c. That believed not; he sets forth the Israelites unbelief, as the original of all their disobedience and rebellions, and the great cause of their destruction. See Hebrews 3:17-19 4:2.
Related Commentaries of Jude 1:5
Jude 1:5
5 I will therefore put you in remembrance, though ye once knew this, how that the Lord, having saved the people out of the land of Egypt, afterward destroyed them that believed not.