The trouble with defining religion as anything you want to believe is that it ends up including a lot of things that common sense would tell you is not religion. It could mean just about anything that ends with an "ism:" captialism, socialism, facism, idealism.
There have to be some criteria, or the word is meaningless. The 20th century theologian Paul Tillich defined faith as "the state of being ultimately concerned." In other words, concerned with death and life after death. So one of the main criteria for religion in his view was that it deal with "ultimate concern." That might be a good starting place.
I think religion also has a social aspect. So there should be a mechanism to share the belief system with others and pass it on to other generations.
These are just a few ideas off the top of my head. Obviously defining religion is a bit more complicated. But I do think for the concept to have any value, it needs definite limits.