His central argument in the book goes something like this:
(1) the fact of vast global religious diversity.
(2) the fact of cultural, historical, and geographical dependence of religious belief.
(3) the mutually exclusive nature of many, or most, faiths.
(4) there's a low probability that whatever your largely culturally determined faith happens to be is the correct one and
(5) a disposition of informed skepticism toward one's faith – the same sort of skepticism members of faiths typically have toward other faiths – should be adopted in order to best determine which, if any, faith is the true faith.
The actual test itself is not terribly controversial, it seems to me. Most believers who were raised in a particular religious tradition, as they grow older and are exposed to a wide world of conflicting opinions and claims, begin to question what they were taught, and go on to apply skeptical scrutiny to those beliefs and teachings to the best of their ability. Most are forced to face, to some degree, these facts and apply this test at some point, whether they realize they're doing so or not. And for those who don't, they should.
However, even this brand of natural skepticism is not enough for Loftus as he demands the skepticism be of a narrowly circumscribed, scientific bent. He does so, he claims, because science has proven a tool of unparalleled success and fecundity at establishing truths about the physical operations of the universe. But why ought we assume science is the appropriate tool for assessing a question that fundamentally is not a scientific one, namely which religion is the true religion? The tools of science can conceivably aid us in assessing specific claims of supernatural activity in the present-day, and so can, in some manner, contribute to the discussion. But it's a tool that's hardly all-sufficient for the task.
Further, he introduces a false dichotomy when he appeals to the problem of mutual-exclusivity. Namely: either you are a sectarian exclusivist (for whom the problem applies in full), or a syncrenistic liberal who alleviates the difficulty but only by defining God down to some milquetoast non-entity. But he neglects the fact that religions have ways to solve this problem without succumbing to this dichotomy. As an Orthodox Christian, for example, I believe my faith exclusively represents the fullness of the apostolic faith as bequeathed to us by Jesus Christ, but also believe that all other genuine religions possess various degrees of that truth. Roman Catholics possess a greater degree of it than Buddhists, and Buddhists more than atheists, but there is a continuum of truth. This schema – and I'm sure other religions address the problem in a similar way – obviates the problem of mutual-exclusivity and so reduces the need to adopt a stance of severe skepticism. He would no doubt object that these alternative schemes do nothing, in and of themselves, to help answer the question of which is the true faith given his preferred set of data. To which the answer, of course, is: so what?
Another question that arises is why the facts of religious diversity, dependence, and mutual-exclusivity should be the starting point for inquiry as to which religion is true? If we accept that they ought to be, then Loftus' recommendation to adopt a mode of severe skepticism mostly (with some qualifications) follows. But in order for this to be one's starting point of inquiry, one must attempt to strip away everything else that he knows to be true in the arena of religion. If, for example, you have solid evidence that that none of the antique polytheistic gods or god-schemes are true, and you further have strong metaphysical arguments both for the existence of God and that God must be One (to take just a couple of things you could conceivably know), then you can rationally narrow the range of contenders for the one true religion considerably. This is an alternative beginning point of inquiry, in which case the probability that one of that remaining small pool of potential gods is the true God is substantially larger than it would otherwise be, and severe skepticism would not be warranted. Loftus demands we bracket out this sort of knowledge and begin with only the facts of religious diversity, dependence, and mutual-exclusivity, but why? Loftus insists that we attempt a Descartes-esque feat of discarding all our beliefs and knowledge in favor of a couple of his pet data points. But why? He doesn't make a case for this preference – for these particular facts over and against the vast array of other facts available to us – other than that they happen to better serve his agenda.
The answer to the question of "why the OTF?" he gives later in the book, and it's that "what we've been doing isn't working." By which he means people aren't becoming atheists, or aren't consolidating their view of God, or aren't all converting to the one true religion (whatever it may be) at a fast enough pace for his taste. Which, of course, begs the question. Why would we expect to see any of these things happen if a particular faith was true? Why does their failing to happen constitute "not working"? According to what standard?
And even if we wanted to begin with Loftus' preferred set of data, is it even possible for an insider to apply the test? Late in the book Loftus reveals his hand: "Faith is not something Christians can have while seeking to examine the religion that was given to them, since that is not how they approach any of the other religions they reject." In other words, you must first become an atheist (since a Christian without faith is an oxymoron) before you can truly examine the truth of your own or other religions. You must actually become an outsider first, not merely do your best to examine the evidence as if you were one. But this is clearly absurd, for whatever epistemological ground one currently occupies, one is an insider to it. There's no escaping except to some other insider position.
Loftus has an unfortunate penchant for repetition and one of his most oft-repeated assertions is that "Possibility doesn't matter, probabilities do." After the 30th time (literally) he had declared this, or some close variation, I gave up counting. In any case, his point is that absolute certainty does not exist, and the scientific mindset only deals with probabilities. Things that are highly likely to be true and things that are highly likely to be untrue are where the most certain knowledge we can attain (about the physical world) is. Again, not terribly controversial, so I'm not sure why the incessant repetition. The cynic in me suspects some sort of insecurity, but I digress.
Loftus' blind spot, with regard to his probability point, is that, while low probability that a particular faith is true logically follows from his premises – given his arbitrarily selected set of data – this low probability applies to all opinions about the religious question, including the stance that all religions are likely to be false -- his stance. Given the diversity of opinions about religion, their cultural dependence, and mutual exclusivity (and only those facts) the likelihood that any opinion on the matter is correct is very low, including his own. He might then appeal to other facts and knowledge in order to justify his not adopting a stance of extreme skepticism toward his own view, but religious people can do the same thing. So he hasn't advanced the ball a step.
In the book he quotes Christian philosopher Alvin Plantinga making a similar point:
[T]here is no safe haven here, no way to avoid risk. In particular, you won’t reach safe haven by trying to take the same attitude towards all the historically available patterns of belief and withholding: for in so doing you adopt a particular pattern of belief and withholding, one incompatible with some adopted by others. You pays your money and you takes your choice, realizing that you, like anyone else, can be desperately wrong. But what else can you do? You don’t really have an alternative.
Precisely. It's this critique and some closely related objections that Loftus spends most of the second half of the book attempting to ameliorate the effects of, but without any success. In response to Plantinga, without so much as a hint of self-awareness, he avers that "Plantinga fails to understand the huge difference between assenting to a belief and doubting it or denying it. There is no epistemic parity at all between accepting a belief and doubting (or rejecting) it. Doubting (or rejecting) a belief is easy. We all do it all the time. The hard part is to set forth a positive case on behalf of any one particular truth out of the choices available." But Plantinga is obviously making the point that rejecting one belief – especially the particular species of belief that belief in God is – logically entails holding another belief (if not a whole array of other beliefs). In this case, rejecting belief in any religion entails the belief that all religions are very likely false. There exists no neutral epistemological vantage to occupy.
Numerous times throughout the book Loftus accuses believers (usually falsely) of special pleading on behalf of their religion. But in response to these sorts of critiques he special pleads on behalf of atheism or skepticism constantly. When speaking about the cultural dependence of religious belief he cites statistical global maps of religion that show high concentrations of religions by geographic area (as if this could alarm or surprise any informed believer). When responding to the objection that atheism is similarly geographically situated – heavily in places like Sweden and Denmark for example – Loftus gets to the special pleading. He does so by citing wide agreement between scientists on scientific matters, regardless of where they're from. But this would be akin to a Roman Catholic citing broad agreement on many central matters of faith, regardless of where the Catholic lives. It doesn't actually address the critique but moves the goal posts, and equivocates between atheism and science. The mechanical philosophy of the Enlightenment is not a neutral, necessary outcome of science and reason, rather it's just as much a culturally and historically contingent source of belief as being born into a Christian home in the 21st century. And, if you live in much of modern Europe, it's at least as big a culturally determining factor of your beliefs about religion as Christianity is.
Loftus responds to one of his interlocutors on a similar point by declaring that "If he thinks for one moment that, as an outsider, I must take an outsider stance to an informed skepticism based in science and reason, then he needs to show why the science I base my argument on is faulty." Must Loftus first exhaustively demonstrate why each believer's particular faith, or epistemology, is faulty before suggesting they submit to the OTF? Not according to him. He claims the facts of diversity, dependence, and mutual-exclusivity makes this incumbent on any rational believer, irrespective of whatever defenses they may have for their particular faith. Yet these same facts do not make the same thing incumbent on him: one must first knock down his position, then demand he take a stance of skepticism towards it, while the believer must avail himself of no defense before adopting the same stance. Again, he’s special pleading.
Another critical problem with this text is the explicit faith-reason false dichotomy that runs throughout, and comes to a head in the final section. He conceives of faith and reason as diametrically opposed and competing epistemologies. This is fine when preaching to the choir of atheists who share this understanding of ‘faith’, and while remaining incurious about what faith actually is in its essence. But since the stated purpose of the book is to propose a test for believers to apply to their religion and determine whether it's likely true, it would be of more significance if he examined what ‘faith’ has been considered to be throughout history, and across cultures, or at least what current believers – his main audience – understand it to be, and what its relationship to reason is. If he did so, he would see that he's attacking a straw man.
He would further have to acknowledge that, on this proper understanding of faith, it isn't optional. Every act of reason always-already entails an act of faith. When some of his interlocutors (mostly Christians who tenaciously comment on his blog, apparently) point out that there are many things we do not know by way of scientific or rational inquiry, such as that there is a material world, that we aren't currently in a virtual reality, that our senses are reliable etc. rather we must accept these things on faith, he replies:
Christians retort that I have faith in reason, in skepticism, in science, in my senses, and in the evidence, but what can they possibly mean? Could trusting them be conceivably wrong at times? Yes, of course. We even know this. But there simply is no alternative but to trust them.
He further objects to these sorts of objections by, once again, reiterating that we must go on probability, and the probability is extremely unlikely that, for example, the material world doesn't exist. But what tests can be devised to assess the probability that we do not currently reside in a virtual reality? Or that you didn't pop into existence 5 minutes ago with all of your memories intact? By the very nature of these sorts of problems, the probabilities can't even be assessed at all and so appealing to probability is a nonsensical move. He's right, of course, that we can do nothing but proceed as if our senses can be trusted and as if the material world exists. But to claim that this doesn't constitute a faith commitment requires justification, and he gives none.