I want to remind you of something you already know well: that the Lord, after rescuing his people from Egypt, later destroyed those among them who refused to believe.
In context
3Dear friends, I had every intention of writing to you about the salvation we share, but instead I find I must urge you to contend for the faith entrusted to the saints once for all. 4For certain men have slipped in among you unnoticed—people marked out long ago for this very judgment—ungodly men who twist our God’s grace into an excuse for immorality, and who deny Jesus Christ, our only Master and Lord. 5I want to remind you of something you already know well: that the Lord, after rescuing his people from Egypt, later destroyed those among them who refused to believe. 6And the angels who did not keep their own domain, but abandoned their proper dwelling—these he has kept in eternal chains, under darkness, to await the great day of judgment. 7In the same way, Sodom and Gomorrah and the surrounding towns, which plunged into sexual immorality and pursued flesh of another kind, stand as a warning, paying the penalty of eternal fire.