CG, what matters is how you use the word, not how it is conceived. Irrational, in this context, is used to denote that religion may not be based on facts - after all, if it were, it wouldn't be a faith. Calling a religion rational, then, would mean you believe that religion stems from a source whose existence is undeniable.
(At this point, I ask myself if a religion's rationality depends on its irrationality, and really, there is no way to prove a religion. This is the main reason I find agnosticism to be the most rational, as it accepts uncertainty.)