Why shouldn't practice irrational things anyway?
Clearly, there are some difficult questions that both Theists and Atheists are asked to answer. There are two points both parties are fairly evenly matched at. One is the beginning of the universe or first cause problem. The second is the mind body problem.
The Theistic position is irrational only in so far as it proposes a supernatural event takes or took place rather than a natural one. In the world we observe, there are no supernatural events, so we can infer that a supernatural event did not once occur (at the dawn of time), to then never be seen again, or that it magically works on one place (within the mind) and no where else in nature. As always, the position of the Atheist relies on probability and rational inference, rather than extreme possibility or a commitment-to-a-deductive-proof-or-anything-goes mentality. Am I being cynical?
A Theistic response does lend a helping hand to physical problems we have trouble comfortably explaining, such as the mind body problem, or the problems we have fathoming energy and matter truly never being created or destroyed (emphasis on CREATED).
To be frank, it IS hard to imagine that the universe has simply existed, with no starting point. And the problems that come with the mind body problem are far too numerous and complex to really get into here. The Theist may find some comfort here; however, I feel it is short lived. Primarily theists almost always belong to some religious sect. This is THEIR downfall.
A real irrational jump comes in when we talk about the transition from theism to Christianity. Even IF we wish to accept that theism must be accountable for some aspect of our being, ANY answer involving a story about God, (Christianity, Judaism, any Islamic tradition, the Greek Gods, the Roman Gods, the Flying Spaghetti Monster) all seem equally as likely, given you can invent ANY story about your God to fit in nicely with the observable world. Though it seems easy to by into a theistic position, any position you take beyond that is pure extrapolation.
I think that I ought to say that I am an Atheist, because, when I say that I cannot prove that there is not a God, I ought to add equally that I cannot prove that there are not the Homeric gods. - Bertrand Russel