Repetition is one of the devices used in the Bible to help Christians realize the seriousness of God’s warnings and instructions. Though the church would be familiar with the history of the Exodus and the other events which will be listed by Jude, he now refers to them as examples of people who sinned and were subsequently judged by God. Alternatively Jude may be speaking apologetically, sorry to remind his audience of something they already know, or rebuking them for not acting on the basis of what they already know.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.