I’ve heard the Course referenced in passing many times, and assumed it was just general New Age teachings. It’s not. At all. It’s devoted to training your mind to perceive that everything around you is an illusion and we are actually extensions of God who are caught in a self-created delusion.
This rabbit hole started with cults, which are a perennial fascination for me. There is a whole internet community devoted to exposing and explaining fundamentalist Christian groups, and also helping people leave them.