Are most philosophers of religion theists? You may be right, but I don't actually know. And I also don't know whether most philosophers of religion are theists before they study philosophy of religion. I also don't know how many philosophers change their minds after they study philosophy of religion.
My sense is that what you're really interested in is how much influence philosophical arguments have on people's religious beliefs—at least, if the people are people who study philosophy. It's an interesting question and all I have to offer are personal impressions, which may well be wrong.
I'd guess that there are lots of philosophers who stopped believing partly because they studied various arguments for the existence of God and found them to be inadequate. I'd guess that because it fits a fair number of people I've known, but as they say: the plural of "anecdote" is not "data." I'd also guess that there are far fewer philosophers who started out as non-believers and became theists because of their study of philosophy of religion. The main reason I'd make that guess is that I haven't met many (if any) theists who became believers on account of the arguments, but once again, anecdotes don't add up to data.
A bit more "philosophically," my sense is that there aren't many, if any, good "proofs" for the existence of God. I say that having given many of them quite a bit of thought, and having co-authored a textbook on philosophy of religion. Further, the highly anthropomorphic idea of God that many believers have in mind strikes me as not even wrong, to borrow a phrase attributed to Wolfgang Pauli. My sense is that for many philosophers, that's the end of the discussion, but I don't think it should be.
One reason is that highly anthropomorphic notions of God aren't the only ones on offer. Another is that sophisticated theological claims may not be your cup of tea, but they don't strike me as a whole lot stranger than some of the ideas that get taken seriously by serious philosophers or, for that matter, serious physicists. But perhaps the main reason is that religious belief isn't, never was, and mostly doesn't pretend to be a purely intellectual matter. The outlook of many believers is much more a matter of how the world feels to them or strikes them in a hard-to-articulate way, rather than a matter of proofs. To see religious commitment as mainly about arguments and reasons is a bit like thinking one has to have good arguments for loving the ones one loves.
This is not to say that religious beliefs are beyond critique; religion can be at least as toxic as love can. Still, the fact that there's bad love doesn't prove that love is bad. Quite apart from that, however, philosophy of religion that doesn't stray beyond arguments for and against metaphysical theses is ignoring the biggest part of the picture. Not clear why a philosopher would want to do that.