http://debunkingchristianity.blogspot.ca/2015/01/some-new-arguments-agai...

(2) The Christian philosopher Alvin Plantinga has argued that religious pluralism is self-defeating. Whether he's right or not, I think religious exclusivism is self-defeating: by accepting that one and only one religion is true, one undercuts the very foundation of religion. The argument goes like this:

(i) Assume that religious exclusivism is true, and that one of the world’s religions is correct.

(ii) Most religions in the world are founded on revelations that self-professed prophets reported having. This includes the major world religions, such as Judaism, Christianity, and Islam. Over time, thousands or maybe even millions of self-professed prophets have reported that they received “truths” directly from the supernatural. The cases of the Apostle Paul, John (of Revelation), and Muhammad are not unique.

(iii) Assume that the large majority of these prophets were sincere in their reports. That is to say, they really believed they communicated with the supernatural. Only a small percentage of self-professed prophets have been duplicitous charlatans – perhaps Joseph Smith is a likely instance of this.

(iv) It follows from (i) – (iii) that some 99% or more of all the self-professed prophets who’ve ever believed they communicated with the supernatural actually did not. The prophets of one’s own religion thus fall into the 1% – or perhaps even the 0.1% – of self-professed prophets who weren’t utterly deluded, and wrong.

[click on link for the rest of the argument]