Would you go up to a Buddhist and say "What if Nirvana is simply a fantasy you have after prolonged psychological stress?" Or go up to a Christian and say "How can one have free will without moral agency and self ownership? What if this whole repentence thing just inhibits your spiritual development?" Or to a Muslim and ask "What if you die and find out Allah is actually a woman?"
Or here's a real kicker what if you're a theist and question if God exists or an Atheist and question the assumption that he does not exist? See it's all well and good to question someone else's religion but God forbid you question your own. And if you start questioning someone's religion odds are they'll get very upset with you because you are questioning the underlying beliefs they've based their life on.
As I said, it's not easy to question someone's faith, especially if you ask such questions, which go against the deep beliefs. But what are you looking for by asking a controversial question? Do you really think that the person is going to tell you, yes Allah is a woman ? He won't and will probably be upset by your question. But the fact is that he'll probably think about it by himself, and therefore question his faith somehow, in a personal way. Moreover, what's the problem with asking God existence, when you are a theist ? And question the assumption for an atheist ? I don't think that the Manichaean view is appropriate to these questions. One can always find nuances and intermediaries. In fact, it's quite normal to ask questions, and if God exists, he is not forbidding anyone to do it ! Let us take the fact that a theist questions his faith with what he has been able to see and hear and then decides to no longer believe in the same way as before or even no more at all. Would God be furious and vengeful ? There would be no point do that, because the mere fact that there is no undeniable proof of its existence, proves that questioning is allowed !