Jude’s reference appears to come from Numbers 14:11–12 where the Septuagint (LXX) uses the same verbs as those found in this verse. Judgment on the Exodus generation is often recalled in Scripture (Numbers 14:26–35; Numbers 26:64–65; Psalm 78:21–31; Psalm 106:25–26; 1 Corinthians 10:5; Hebrews 3:16–19) with the root cause identified as unbelief (Numbers 14:11; Psalm 78:22; Hebrews 3:19). In the context of the Exodus narrative, to be destroyed meant that the people were not allowed to enter the land of promise but died in the wilderness.1
5 Now I want to remind you, although you once fully knew it, that Jesus, who saved a people out of the land of Egypt, afterward destroyed those who did not believe.