Faith and "Truth vs Utility"

[Valyza1]

I have often heard people say that the only good reason to believe something is if it is shown as likely to be true. What if, however, you have a proposition for which it is impossible to show evidence either for or against it's truth value, but also for which there is great utility in adopting? Is adopting this kind of proposition as if it's true just as good as (if not even better than) adopting a proposition that is demonstrably true? If the proposition is "God exists", I think many theists might answer yes to the question, but I'm not sure.