If the existence of any non-empirical entity is irrational, how do you decide what the right thing to do is? When you get to a fork in the road, how do you decide which way to go? (In philosopher-speak, how do you derive normative statements from exclusively positive empirical statements?)
Faith is a lot trickier than religious belief and can include untested but not untestible propositions (such as "I believe going right will get me to place X more effectively"). It's still a faith claim, but of quite the different sort. A number of scientific theories and axioms fall under that umbrella. One of the larger issues with a great deal of metaphysically motivated faith claims is that they tend strongly to fall under the latter category -- you have no means of testing (and perhaps more importantly, communicating or replicating the tests you do make). So, you don't need untestable faith to decide which way to go, you just need have prior knowledge of, say, a map and extrapolate from that which direction to take.
There's also plain random chance, or simply choosing without sufficient justification for discrimination -- in the former case, you let some (sufficiently) random principle (coin toss, RNG code, whatev') choose for you. In the latter case, it's a simple principle -- if you have no reason to favor one choice over the other, either will do. The old philosopher's example is with an animal set before two piles of food, of equal size, quality, and distance from the animal, with no other influences on the animal's choice. Tellingly, the animal will not starve because it cannot choose between two choices in all ways equal, and neither will humans. You choose one and go along.
Just as the parallel postulate (or its negation) lets you make lots of interesting conclusions about space, belief in god (or any other normative principle) lets you make lots of interesting conclusions about the rightness or wrongness of certain behavior. Unless you are willing to adopt some faith-based belief (christian god or no) how can we know how to act?
Belief in god isn't a normative principle -- belief in a particular religion's moral doctrine (Particularly regarding the implications of that god's existence) might be, though.
As for normative beliefs, my preference is aiming for whatever has the least metaphysical burdens, if for no other reason than that less moving parts means less chance of breaking. Moral good does not require divine mandate, and can be rationalized by an appeal to social structure or self-interest (or a few other things as well, really). The simple answer is that there's not many conclusions reachable through religiously motivated normative beliefs that can't be reached by ones that aren't so motivated (and certainly none I've thus far been interested in holding)-- and in which case, why complicate things needlessly? Especially when there's a historically repetitive occurrence of needlessly assumed metaphysical burdens contradicting or interfering with themselves

I just kinda' like to start
and stop my normative belief formation with "Be awesome to one another as much as possible," basically. Because I'd like you to be awesome to me, and assume that's reciprocal. When folks don't let their burdens get in the way of that, well, we're cool, y'know? We don't actually need more than that if everyone'd stick to it.
Talking about religion on the net never goes well, but believing in god myself, it's interesting to see why others lack faith.
I'm just going to leave a note here-people say the church has done so many terrible things in its past (crusades, inquisition, etc.) but what about all those benevolent services done by religious institutions? The Salvation army is a good example, or the smaller charity activities done by local churches.
I think the same about those as I do about non-religious charities, m'self. Good works are good works, regardless of metaphysical burden or lack thereof. Some of them are discriminatory based on religious beliefs, though, which kinda' highlights the
interfering metaphysical burden thing.