Usually a cult claims "new revelation", usually through a self-proclaimed prophet. Frequently it re-interprets scripture, or adds writings that are extra-biblical. And invariably they teach that the old, established faiths are being replaced by God (or other deity) to theirs. Invariably, theirs is the only "true" religion. Sadly, millions have fallen for them.
Such cults will be judged, having pulled people away from truth to follow a lie. Revelation tells us about them Chapter 22:18 "I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book; 19 and if anyone takes away from the words of the book of this prophecy, God will take away his part from the tree of life and from the holy city, which are written in this book."
That will be the eternal lot of many.