It is not only Christians who do this, though. I have heard Jews argue that their religion must be true because it is the only one in which god spoke to a huge number of people at the same time and thus they could not all be lying or deluded. Their source for this claim? The Old Testament.
I also had a discussion with two Mormon evangelists who came to my door. They claimed that the Book of Mormon must be true because it correctly predicted things. When I pointed out that the book was written after the events that it allegedly ‘predicted’, they disagreed saying that the book was written before the events but was discovered by Joseph Smith after. Their evidence? The Book of Mormon itself.
It is kind of amusing demonstration of how the desperate desire to believe can result in people abandoning their reasoning skills.