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How often have you heard the following sort of argument - "Given my assumptions, it's perfectly rational to believe in Xianity (or insert any religion here as YMMV)."
But, is that really true? I mean, using this as a guideline, anything could be considered rational if one uncritically accepts certain assumptions.
I simply have to have the right assumptions, like that unicorns exist, and they are invisible because they are magical creatures and can do things like that. I'll also assume that they don't love us, and would like to see us enslaved. I'll also assume that they have an unhealthy obsession with humping other animals and are big fans of genetic manipulation, yet lack the ability to use test tubes (due to having hooves instead of opposable thumbs) so they have to use eugenics instead of manipulating genes.
Given these assumptions, it's rational to believe that invisible alien unicorns are secretly infiltrating petting zoos all across the nation and having sex with the animals in a secret eugenics-type program in order to breed animals that will rise up and overthrow their tyrannical human overlords and install the unicorns as leaders of this bountiful planet.
The problem, of course, is that uncritically accepting assumptions that are unsupported is not rational. So, the acceptance of irrational assumptions puts the final conclusion also in the irrational category. Therefore, it simply is not rational to believe in god, especially due to the irrational assumptions that are required in order to do so.