Which brings me to the core difference in our thinking: you hold that "it seems a bit silly to me to embrace a belief system for reasons other than that it is factually true". I understand this clearly. But I don't share it. As I wrote, over the years I have ceased to use the word "truth", as "I began to perceive, as another Finnish theologian Ari Hukari has recently done, that TRUTH is merely a matter of a chosen language-game, and therefore limited in its usefulness".
I think you represent mostly the school of scientific realism, while my affiliation reaches the other end of the spectrum (the "neopragmatist" school). This seems to me to be evident enough in your wish to talk about truth values, and my wish to talk about issues without this concept of (absolute) "truth" that has been rendered meaningless in my journey through the field of contemporary philosophy.
Over the years, confronted with many anomalies in the study of history (in the methods historians use), I became persuaded, especially by various neopragmatists, that the clear-cut realist view of the science and argumentation - although a good heuristic tool for actually existing as part of modern society (and enjoying the benefits of computational innovations, thanks) - couldn't be philosophically backed up: in the contest of arguments the postmodern section won.
You will not be surprised when I say that my idea of rationality differs from yours
Bah, postmodernist malarkey. It's basically a wordy cop-out. And by saying that his idea of rationality is different, he's basically trying to give himself a loophole to get out of having to make an argument.
All I can think is to ask him point-blank what his beliefs are. He will probably tell you he can't actually back them up with reason, because his idea of reason is different or some such. If he doesn't have a reason to think it's true, even in a wishy-washy postmodernist sense, then he loses, plain and simple. It will probably come down to "just 'cause," and at that point there really isn't anything you can say except "goodbye."
I didn't realize just how crazy people could get...maybe it's best to just let the sheep be sheep, fleece them and let them graze in peace. I sympathize with your frustration.