I guess the question here is "What does it mean to be rational?" Is there some "objective rational right" that we all must follow in order to be rational, or is it something more subjective. I contend that what is rational for me may not be rational for you. We both are different people with different backgrounds. There is no way for me to expect that you can, or even should, belive the same things I do. Perhaps it IS rational for me to believe in God based on my experience, or my context. No two people are the same or have the same experiences to base their rationality, their beliefs on.
It seems like we run into trouble when we assume that we should all believe the same things. Obviously you cannot argue with a Christian in the context of Christianity if you are not a Christian. You can both be "right," that is, both be behaving perfectly rationally, and so any argument is very illogical.
Its important to remember, however, that just because rationality may be relative, truth is not necesarily subjective. Though you may be perfectly right in beliving what you believe, what you believe may not necesarily be the truth. This is where agnosticim kicks in. If we are all perfectly entitled to our beliefs (as long as we are intellectually virtuous in obtaining those beliefs) who is to say which belief is right? It is wrong to attack anothers beliefs without first admitting that your beliefs are on just as shaky grounds as any other. Your believing them makes them true for you, but does not necesarily align them with Truth.