Most religions around today seem to be based on a 'sacred text' (ST) of some sort (Bible, Koran, Book of Mormon etc.). This is assumed to represent 'The Voice of God' and to be the ultimate source of authority. Most religious 'fundamentalists', that I have met, can't seem to get beyond their particular ST when advancing their 'arguments'.
In his recent book 'God is not Great' Christopher Hitchens examines some of these texts and concludes that there is a very high probability that they are all man made and are often of dubious provenance. I am not at all surprised by this conclusion. Surely, all you need to say to a fundamentalist, on your doorstep, is: "I'll listen to what you have to say if you can convince me that your ST is really 'The Voice of God' and not the work of man".