The question that seems to have generated the most interesting results for me in conversations with Christians is "On what basis should a Muslim deconvert?" The power of this question is that it forces you to consider what would make you deconvert from a religion, while separating you from the emotional ties you have with your own. It implicitly asks you if you hold yourself to the same standard you hold other religious believers (who you presumably think have the wrong idea about God, and should convert).
I've been surprised by how many Christians have answered this the same way I did three years ago, but arrived at a totally different conclusion. My answer was that there was no standard I could apply to both them and myself that would declare my religious experience to be true and their religious experience to be false. I had to admit that, had I been born a Muslim and maintained my current standards for belief, I would have forever remained a Muslim. What surprised me about the Christians who echoed my sentiments was that their response was not to doubt the validity of their belief. Rather, their response was to thank God that they had been born into a Christian environment. Several times I have heard the phrase "if I had been born a Muslim, I think I would probably still be a Muslim", and was astonished that it was not immediately followed by a recanting of Christianity.
This seems so logically inconsistent to me that I don't really know where to start. This is a confession that your beliefs are not based on reality, but on Geography. This is a confession that your beliefs are not objectively true, but rather culturally convincing. This is a confession that your standards of belief are so weak that you cannot even differentiate yourself from your principle rival (globally speaking) that claims to offer a contradictory version of truth. And most important, it seems a confession that there is no reason anyone should choose Christianity over Islam, because their relative plausibility is based on your culture rather than the truth. I just can't get my head around someone admitting to all this, and being thankful they were born into this particular arbitrarily held belief system- as if the Muslim would not say the exact same?
I honestly think Islam is the best argument against Christianity. It falsifies a lot of the core assumptions of Christian apologetics- that a group of men two thousand years ago wouldn't have died for a belief unless it was real, that God reveals himself in an active and unmistakeable way, that miracles and fulfilled prophecy are convincing and sufficient to prove a religion correct. Moreover, it shows that people can be absolutely convinced that their religious beliefs and experiences are true- but be totally wrong.
My point is, if you don't have a good filter that passes your religion and fails every other religion, then I think you need to reevaluate your standard of belief. This is why I placed such a premium on the Personal Relationship claim made by Christianity, because it was just one such differentiator. But in the end, that didn't bare itself out in my life.
I'd be interested in hearing any suggestions about what kind of filter we might apply that passes one and only one religion- or an argument of how we're justified believing one of the two religions without such a filter