As a believer, I think that theism is more reasonable than atheism although I
I hope you are right for I while I am a Christian philosopher (or a philosopher who is a Christian) I believe that many of my friends and colleagues who are atheists or agnostics or who accept Islam or a non-theistic view of God (as my Hindu philosopher colleague and friend) are just as reasonable as I am in the sense that each of them has intellectual integrity and has spent at least as much time intelligently reflecting on their convictions, earnestly seeking the truth in such matters. Still, I think each of us needs to hold that the reasons that justify our different beliefs are not defeated (undermined) by the reasons for incompatible beliefs. An atheist might be able to acknowledge that I am just as reasonable as she is, but she cannot (in my view) think that her reasoning is undermined by the evidence or reasoning that I undertake. Alternatively, consider a Christian-Muslim exchange (something I am deeply committed to). I accept a traditional Christian understanding of God incarnate on the basis of an historical argument and an argument from religious experience (I roughly following the reasoning of the Oxford based philosopher Richard Swinburne in his book on the incarnation). In doing so, I believe that I am committed to thinking that no one had decisive, irrefutable evidence against the incarnation that any reasonable person would or should accept. I can certainly recognize that my Muslim philosopher friend Mohammad is reasonable in only recognizing Jesus as a holy prophet (peace be upon him), but there is a limit here in terms of my not being able to accept that he knows (with certainty, based on irrefutable evidence) that Jesus was only a prophet.
Three other points are worth noting.
First, I believe that the above matter is not special to philosophy of religion, but it runs throughout metaphysics, ethics, epistemology, philosophy of law, philosophy of language, science, and so on. I have two colleagues that are Kantian and one that is Humean. They both cannot be right about the nature and normative status of ethical obligations, but as far as I can telly they are intellectual peers and regard each other as equally reasonable.
Second, it is partly because we do (in the practice of philosophy) believe that colleagues who disagree with us are equally reasonable that we are motivated to engage each other in debate and sustained arguments. Without that assumption / premise, the very landscape of philosophy would look more hostile (in my view) than it currently does.
Third, as a general point, I happen to think that the reasons why philosophers adopt the positions they do is highly complex and historically conditioned. My hypothesis is that philosophers form their views on different matters based on clusters of arguments, their view of certain concrete cases which they interpret differently in light of alternative theoretical commitments, the success or failure of thought experiments, their particular exposure to positions during their graduate education, and perhaps even psychological and sociological reasons. For example, one person might naturally rebel against the perceived status quo which is why he or she adopts a form of phenomenology in a department which is structuralist, whereas another person is an anti-realist about freedom in a philosophical libertarian culture. So, in offering this third suggestion, I suggest that we rarely have a case in which two philosophers disagree about X because they disagree about the evidential force of a single, separate line of reasoning. To give a concrete case, I think Philip Kitcher is just as reasonable as me or probably more reasonable than me philosophically (he is older, has been practicing philosophy longer at an elite university, while I am a mere College professor). I accept a cosmological argument for theism (you can find a good version in the Stanford Encyclopedia of Philosophy), whereas he does not. Imagine we read the same article. The reasons for our diverging in our assessments probably lies outside of the line of reasoning in the contribution. Kitcher, for example, adopts a form of pragmatism when it comes to ostensibly necessary truths that I think is mistaken. For us to debate the cosmological argument, we would probably need to debate the adequacy of his pragmatism, and then probably move on to ever greater areas of epistemology and metaphysics. Overall, then, I suggest (going back to my original response) two philosophers may be equal in intellectual integrity, equal in focussed, intelligent reasoning, equally in identifying the truth or most reasonable position(s), and yet reach divergent views, partly due to the highly complex, interwoven nature of philosophy.