5 Now I desire to remind you, though you already know this, that the Lord, having saved a people out of the land of Egypt, afterward destroyed those who didn’t 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.