God judged Egypt. He judged them severely for refusing to let his people go and worship him in the wilderness, and he judged them with increasing severity, until finally the firstborn of every Egyptian family died from the firstborn of Pharaoh to the firstborn of the captive in the dungeon.
Wouldn't that be an assurance to the faithful? – God's promise unfolding in the future. That is one of the functions of prophecy in Bible times, to be an assurance to people, as things happen.