Jude 1:5

Having spoken about the debauchery of the impious Nicolaitans, Valentinians, and Marcionites, he adds also that the Lord delivered the people from the land of Egypt, and so on. By this he shows that the Founder of the Old and New Testaments is one and …

He afterwards destroyed - The far greater part of that very people whom he had once saved. Let none therefore presume upon past mercies, as if he was now out of danger.

The first ten verses of that chapter (as the scripture is always the best commentary upon itself) are the best explication of the fifth verse of this epistle of Jude. None therefore ought to presume upon their privileges, since many who were brought out of Egypt by a series of amazing miracles, yet perished in the wilderness by reason of their unbelief.

I will therefore put you in remembrance, though ye once know this,....

There are many things which we have known which yet we have unhappily forgotten. Is it of no use or service to be put afresh in remembrance of them? Now what are these things which we Christians need to be put in remembrance of?

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.

I will therefore put you in remembrance - That is, how such persons were proscribed, and condemned to bear the punishment due to such crimes. Though ye once knew this - The word ἁπαξ, here translated once, has greatly puzzled many interpreters. It has two meanings in the sacred writings, and indeed in the Greek writers also. 1. It signifies once, one time, as opposed to twice, or several times. 2. Altogether, entirely, perfectly, interpreted by Suidas αντι τον διολου, ὁλοσχερως· and of this meaning he produces a proof from Josephus; This appears to be the sense of the word in Heb 6:4: τους ἁπαξ φωτισθεντας· those who were Fully enlightened. Heb 10:2: ἁπαξ κεκαθαρμενους· Thoroughly cleansed. See also Heb 10:3. Psa 62:11: ἁπαξ ελαλησεν ὁ Θεος. God spoke Fully, completely, on the subject. St. Jude is to be understood as saying, I will therefore put you in remembrance, though ye are Thoroughly instructed in this. Saved the people - Delivered them from the Egyptian bondage. Afterward destroyed them - Because they neither believed his word, nor were obedient to his commands. This is the first example of what was mentioned Jud 1:4.