I mentioned below about how Joe Carter jumped the shark - or really let slip out that it's OK to call non-Christian religions "false religions."
I don't think they were aware of it, but it is indeed along the lines of using the "n-word" in referring to African Americans.
Now, yourdictionary.com has a good definition of religion; there is a theme running through it of beliefs and practices.
Of course, there's religions and there's religions. The theistic religions do indeed have some element of the supernatural, a deity, and a creation narrative. The non-theistic religions tend to have many fewer beliefs (often grounded from personal experience), but many practices. Regardless, the more one engages in a religion, the more one's self changes. This is not simply "spiritual," it is also physiological, psychological, and interpersonal.
So, when one says that someone else follows a "false religion," they are in effect making a statement- and not a very pretty one- about the physiology, psychology, and interpersonal relationships, as well as the spirituality about people who follow the religion that someone ignoranty calls "false." It is another way of saying people who don't follow "my" religion are false, whether or not it is meant.
Oftentimes, though, that is what is meant; hence some Christian Reconstructionists have advocated putting Wiccans and gays to death (over what basically comes down to religious differences.) It is not a very long path from "their religion is false" to "they are false," to "they are a danger to our religion and must be cleansed."
The western world came a long way since the Thirty Years' War showed that wars over religion are immoral bloodbaths. One would hope that we have the wisdom to remember all that death, and try to avoid such things in the future.