Now I'm sure some may find it offensive to have the MBTI compared to religion, but I am going to show you why I've started to see it this way.
Sure, there may not be a God or gods, but not all religions focus on a higher power, some state that the higher power is in us, and talk of seeking a path to better understanding and enlightenment.
When it comes right down to it, what is a religion except for faith in an idea? Well, the MBTI is really just an idea, people are placing their faith in tests and in what they believe to be the "truth" about themselves.
There are many religions, just as there are many types, types that often stick with their own, sort of like protestants, or Catholics, or Jewish people. Sure there may be interaction between the different assemblies. But I have seen so many thread titles and posts that already lump different types together, the same way different religions lump different faiths together.
You have your prophets, your "wise" men. You have your idols.
You have your believers and your non-believers, your atheists and your agnostics, and your faithful.
Maybe because I lack faith, I can not believe in the MBTI.