5 Now I want to remind you, though you know all these things: The Lord first saved a people out of Egypt and later destroyed those who did not believe
The audience that Jude wrote to knew Scripture very well, especially the passages he was referring to.
The people that came out of Egypt were two types. Those who believed in Yahweh, the I Am and those who ended up rejecting God by not listening to those who He appointed as leaders [Moses and Aaron]. God’s promise of the new land [Canaan] was not given to the disobedient, but rather to those who held on to His promises and kept His word.
The word “destroyed” means that physically, the wicked died in the desert and never got a chance to enter into the promised land. But, it also means that spiritually they were lost.