“The argument is that, if your belief depends only on geography and family history, then it has no more chances of being true than the god of the muslim guy, or the hindu guy, or the christian guy who lives right across the border.”
I would like to analyze this statement, and see if it comports with reality. First, we have to ask what is meant by “depends.” I think that perhaps the most charitable meaning we can give this is to say it is being used as “reason.” So, the claim can be interpreted to mean, “If the only reason you have for your belief is geography and family history. . . .” But then “reason” becomes ambiguous, for surely we cannot mean reason as in “rational justification.” This is because most people don’t formulate an argument to the effect of, “I have my geographical and familial situation such that I am pre-disposed to believe in Christianity, therefore, I believe Christianity.” Instead, “reason” tends to mean “explanation” in these cases. So, the claim can be clearly reformulated to read: “If the only explanation of your belief is geographical and familial history. . . .” If this is not a correct understanding of what is being said by this part of the statement above, I welcome a correction. I believe that alternative understandings of this part of the claim will be shown either to be completely or usually false, so that the claim will only affect a minimal amount of Christians (if any). This is why I felt it charitable to read it in the stronger way.
“Then it has no more chances of being true. . . .” In interpreting this part of the claim, we must be careful. Chance is not ontological; it is epistemological. Since someone who has an explanation for their belief in Christianity other than the above (like Christians in Muslim contexts, or ones who come from atheistic backgrounds, etc.) will have many things open to their epistemology, we should locate these epistemological challenges to be relevant to those whom the first part of the statement addressed. If we do not, then the second part becomes patently false (at least, it would be a non-sequitur). This part of the claim states that for those whose only explanation for their belief is geographical and familial concerns, they do not have any more justification for holding their belief as true over and against other beliefs contrary to their religious beliefs.
So, the entire statement would read (in condensed form): “If the only explanation of your belief is geographical and familial history, then it has no more chances of being true over and against other religions, and one is not justified in holding his religious belief.” So is this true? Not at all. For explanations don’t preclude there being rational reasons to believe! So suppose there is the Christian whose only explanation for belief is that he was born in a Christian country to a Christian family. Why can he not have arguments for Christian belief? One can argue that these arguments or reasons are all false or unjustifiable, but it will have to be because the skeptic has examined all of these reasons and concluded they are false, which is a completely different project than the one outlined before. It also doesn’t help to say that many people don’t have these reasons, since many do, and hence it will not work as a principle. It also won’t work to go back to “reason” as rational instead of explanatory. This is because a relative very few take their family history and geography to be rational arguments supporting their belief (much less do they make it the only such argumentative basis). Even if we find some who do, most do not (this is true even if they have no such arguments: most do not retreat to arguing from family history). So perhaps the skeptic can make a synthesis: “If the only explanation of your belief is geographical and familial history, and you have no rational justification (whether by lack or by defeat) for your belief, then it has no more chances of being true over and against other religions, and one is not justified in holding his religious belief.”
Is this modified statement, which affects only those who, a) have no rational arguments or evidence to support their beliefs, or b) those whose every argument and line of evidence has been at least significantly undercut so as not to count as justification, actually true? I think not. This is going to be the point of Alvin Plantinga’s reformed epistemology. Basically, the idea is that even if “regular people” don’t have any rational arguments to support their belief that God exists, they can still be justified in doing so. How? Well, if God exists, it is reasonable to think he accounts for belief in God by placing a sense of the divine into every man. Indeed, something very much like this is the testimony of Scripture (cf. Romans 1).
“Wait a minute!” you exclaim. “You can’t just suppose God exists to show that they are rational!” I’m not quite supposing God exists. What I (and Plantinga) am doing is attempting to show that theists are rationally justified, in the absence of defeaters, in believing God exists. If belief in God is a properly basic belief, which I think it is, then the “man on the street” is perfectly justified in holding them.
Well, why can’t the adherents of other religions say the same thing? Well, they certainly can! However, we must keep two things in mind. First, there is a major difference between knowing something is true and showing something is true. You don’t have to do the latter in order to enjoy the former. Suppose you believe 2+2=4, and you encounter a person who insists, via a complicated and complex formula and set of reasoning, that 2+2=941.6, on one day per millennium. As it so happens, you need not refute this person in order to know that 2+2=4, and therefore, whatever is contrary to it is false. Now, you would certainly need to interact with that man if you wanted to show that his view was false. So you do not need to refute every other religion before saying you know your religious beliefs are true. Second, properly basic beliefs are subject to defeaters. This is how you can show someone their beliefs are false (or, conversely, be shown that your beliefs are false).
In either case, it seems this claim will not work against anyone—unless God does not exist. That may be so (though I don’t think it’s even possibly so, but that’s another argument), but it will be those considerations that do the work. Even then, since epistemology is person relative, if the subject does not know those defeaters, then she can still be rationally justified in holding her religious beliefs (until such time as she has a defeater, of course).
 It’s seriously doubtful whether this is ever the only explanation in that this explanation is neither necessary nor sufficient for Christian belief. It’s not a necessary condition in that someone can become a Christian without ever hearing about a “Christian nation” or being in a “Christian family.” It’s also not sufficient in that there are individuals for whom conversion never takes place, despite being born into a Christian family and in a Christian nation.