It's simply an expression of the competitive trait in human nature. "I am better than you, and mine is better than yours" - you fill in the blanks after that and feel free to include just about anything.
Hey, let's see what "their" god is all about, so that we think of a better one and then call him ONLY.
As far as human nature is behind all that, it can be compared to something like national pride, patriotism, or on a smaller scale, an intellectual favoritism, loyalty to our political party, you name it. Don't forget that "liberalism/conservatism is the ONLY just model of social coexistence." Anybody for dogs on your menu? - you don't know what you are missing.
And there is no lack of the power in our conviction. Why would those guys have such a popular god - we are going to come up with one that you can't quit unless you want to sizzle in fire of hell - and we are not even going to wait till afterlife but will behead those quitters, how is that for a stronger faith?
Even within the same denomination there has to be branching and a competition to draw more numerous congregations.
Indeed, from the viewpoint of a non-believer, there is no business like believing business. "Spreading the WORD" is a very cost-avoiding way of advertisement. In tradition of every business, the product simply HAS TO be presented as THE ONLY REAL ONE - "beware of imitations".