Most of the big religions have a low bar for membership. Islam and evangelical Christianity require nothing more then the spoken word to convert. But even those religions have at least a few rules and rituals you have to half-ass to stay a member.
Enter the Church of Proglotology. Being a prog doesn't require anything at all. All you have to do is call yourself one and, presto! You automatically become a good and wise person in the eyes of other progs.
After that, all you have to do is mouth the right slogans from time to time. And it doesn't matter at all what you do. You don't have to vote. You don't have to give. You don't have to work. Hell, you can even be a complete scumbag and still be in good standing.
Yeah, it's pretty easy to see why it's such a popular group.