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. 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 Hebrews 6:4 : τους ἁπαξ φωτισθεντας· those who were Fully enlightened. Hebrews 10:2 : ἁπαξ κεκαθαρμενους· Thoroughly cleansed. See also Hebrews 10:3. Psalms 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 Jde 1:4.