I started thinking about this because I have, historically, had a hell of a time trying to spell “atheist,” mostly because of that whole i before e thing. I finally got around to remembering by just thinking of it as “The Ist.” This would, of course, make “theism” into “The Ism.” In the current politically correct world, isms and the people who wield them (the ists) are frequently looked at in a negative light. Racism, sexism, ageism, and the like, from which people frequently attach “ism” onto a noun to describe intolerance based on that noun. This is not at all to say, of course, that any word ending in “ism” or “ist” is bad, in fact I am sure that there are more of these words that have either positive or no weight than there are that have negative weight.
So, even though it is just a coincidence, I find it a little ironic that religion and the religious can be described with those words theism and theist. In order to be religious, you have to be prejudiced against all other religions and sects of your parent religion, because by choosing one you are declaring (though often silently) that all others are wrong. Unlike racism, sexism, and the like, where we can overcome those mindsets by educating people, we can’t get rid of religious intolerance unless there is only one or there are no religions. So it really is “The Ism.”