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, respecting the Divine treatment of the wicked in times past.”
Though ye once knew this - That is, you were formerly made acquainted with these things, though they may not be now fresh in your recollection. On the different significations affixed to the word “once” in this place, see Bloomfield, “Crit. Digest, in loc.” The thing which seems to have been in the mind of the apostle was an intention to call to their recollection, as bearing on the case before him, facts with which they had formerly been familiar, and about which there was no doubt. It was the thing which we often endeavor to do in argument - to remind a person of some fact which he 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 - Compare the notes, 1 Corinthians 10:5-12. The bearing of this fact on the case, before the mind of Jude, seems to have been this - that, 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 on 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 - τὸ δεύτερον 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 fixed his mind on this event as a “second” great and important fact in regard to 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 the notes at Hebrews 3:16-19.