Albert Barnes Commentary Jude 1:5

"Now I desire to put you in remembrance, though ye know all things once for all, that the Lord, having saved a people out of the land of Egypt, afterward destroyed them that believed not." — Jude 1:5 (ASV)

I will therefore put you in remembrance. "To show you what must be the doom of such men, I will call certain facts to your recollection, with which you are familiar, concerning the Divine treatment of the wicked in times past."

Though you once knew this. That is, you were formerly made acquainted with these things, though they may not now be fresh in your recollection.

On the different meanings attached to the word once in this passage, see Bloomfield, Crit. Digest, in loc.

What seems to have been in the apostle's mind was an intention to call to their recollection facts with which they had formerly been familiar, about which there was no doubt, and which bore on the case before him. This is similar to what we often endeavour to do in an argument—to remind a person of some fact they once knew very well, and which bears directly on the case.

How that the Lord, having saved the people out of the land of Egypt. (See Barnes on 1 Corinthians 10:6–12).

The relevance of this fact to the case Jude was considering seems to have been this: just as those who had been delivered from Egypt were afterward destroyed for their unbelief (or, as the mere fact of their being rescued did not prevent destruction from coming upon them),

so the fact that these persons seemed to be delivered from sin and had become professed followers of God would not prevent their being destroyed if they led wicked lives.

It might rather be inferred from the example of the Israelites that they would be.

Afterward. to deuteron the second; that is, the second thing in order, or again. The expression is unusual in this sense, but the apostle seems to have focused his mind on this event as a second great and important fact concerning them. The first was that they were delivered; the second, that they were destroyed.

Destroyed them that believed not. That is, on account of their unbelief. They were not permitted to enter the promised land, but were cut off in the wilderness. (See Barnes on Hebrews 3:16-19).