Kevin Brown at Diglotting has looked at the OTF and is somewhat underwhelmed:
So, what's the verdict? Brown concludes:
The OTF can be broken down into a four part argument which I shall summarize as follows: (1) there is lots of religious diversity in the world; (2) because of this religious diversity one can surmise that “one’s religious faith is causally dependent on brain processes, cultural conditions, and irrational thinking patterns” (15-16). These two points lead to (3) “at best there can be only one religious faith that is true. At worse, they could all be false” (16). Thus, the author proposes (4) “The only way to rationally test one’s culturally adopted religious faith is from the perspective of an outside, a nonbeliever, with the same level of reasonable skepticism believers already use when examining the other religious faiths they reject” (16-17).
I do not understand logic of these four steps. While I agree that the religious faith of some people is causally dependent upon their cultural upbringing, considering that fact that many people change their religious faith as they get older, this leaves a segment of society where (2) does not necessarily follow from (1), at least not when it comes to the issue of why one has chosen a specific religion. The second chapter discusses the fact of religious diversity in more detail. In it the author says:
Many evangelicals are exclusivist to a large degree. But this view simply cannot be maintained in the light of the amount of religious diversity in the world along with the subsequent rational disagreement about religious faiths among peers. If there is a God who wants us to believe in him, there would not be so much religious diversity around the globe. The probability that the Christian God exists is inversely proportional to the amount of religious diversity that exists (that is, the more religious diversity there is, the less probable it is that he exists), and there is way too much religious diversity to suppose that he does. (45)
“If there is a God who wants us to believe in him, there would not be so much religious diversity around the globe”. This is representative of some of the logic in this book. If God exists, then X must be true (why? because I say so). X is not true, therefore God does not exist.
For me, I am puzzled that an approach I use to fault-find my theological position can be cited as a theism-buster. Unless one is looking for reasons not to believe.
In a nutshell, this book argues that people should approach their own religious faith with the same level of skepticism that they approach other religious faiths. Amazing that an entire book could be written on this! Unfortunately, the book lacks any philosophical depth (but makes up for it in non sequiturs), is poorly argued, and is primarily aimed towards the exclusivist and conservative faiths (esp. evangelicalism). So it isn’t really useful except for maybe someone who has never given critical thought to their religious beliefs before.