Jude's introductory words were polite (cf. 2 Peter 1:12) but also a reminder that what he now said was fact beyond dispute. His readers knew these things "once for all"because God had delivered them "once for all"in Scripture (v. 3; cf. 1 John 2:20-21).
After God redeemed Israel and liberated the nation from bondage in Egypt, the people failed to continue to believe God's promises and to trust in His power (cf. Num. 14:11; Deut. 1:32). God judged those who failed by destroying them in the wilderness. He let that generation die rather than bringing the unbelieving apostates into the Promised Land. Some of the false teachers in Jude's day evidently were Christians. That is a reasonable conclusion since Jude compared them to the redeemed Israelites. They too were turning from continuing trust and obedience to God, and God would judge them as well.
"This allusion to Israel in the wilderness makes it very plain that Jude's opponents were once orthodox Christians who had gone wilfully [sic] astray into error."30
Other interpreters believe Jude was referring to those Israelites who had never really believed in Yahweh in this verse.31Jude primarily wanted to point out the behavior of these false teachers, not to identify whether they were believers or unbelievers. Unbelief always results in some kind of destruction whether the unbeliever is lost or saved. God definitely destroyed these unbelievers physically. He also destroyed them eternally if they were unsaved.
"Jude insists that the Saviour can also be the Destroyer."32
