Those who preach the truth have nothing to fear from honest examination. If what you're saying is true, you'll stand the test, right? Paul certainly thought so when he wrote to the church in Thessalonica, urging them not to accept anything that happens to land on their doorstep, but to "test all things, hold on to that which is good".
Muhammad, however, didn't want to be tested. He did make the occasional argument, of course, but generally speaking, he expected complete faith, lest the one questioning him should come to a sticky end. One of the less gory examples we find in the Qur'an is Surah 5:101-102, which says "O you who believe! do not put questions about things which if declared to you may trouble you, and if you question about them when the Quran is being revealed, they shall be declared to you; Allah pardons this, and Allah is Forgiving, Forbearing. A people before you indeed asked such questions, and then became disbelievers on account of them." (Shakir).
As a Christian, I have complete faith in the Bible. I'm not afraid that if I question it, I will lose my faith. In fact, my personal experience has been the opposite. I've asked questions, and my faith grew stronger. But Muhammad says if you ask certain questions, you might lose your faith? When I'm told not to ask questions, I feel all the more compelled to ask those questions to figure out what you're trying to hide.
But what about ex-Christians? They asked questions and lost their faith, right? So don't I have the same problem? Not really, because first of all, Muhammad's warning isn't descriptive, but prescriptive. It doesn't just say that some left the faith, but presents this as a reason for not asking questions. The Bible, by contrast, does deal with people who left the faith, but commands you to test all things anyway. What does the Bible do with people who left the faith? It says they were never truly of it to begin with (1 John 2:19). In other words, they're not a problem for us. And indeed, so-called ex-Christians often have a deficient understanding of Christianity. Speaking anecdotally, I find that they tend to have a lesser understanding of Christianity than those who never claimed to be Christians in the first place.
See, there is a difference between asking the wrong questions and asking no questions. An atheist once told me he'd become a Christian if I found the body of Jesus. That's asking the wrong question. He should instead look for historical evidence that Jesus rose from the dead. But I don't expect anyone, whether Christian or unbeliever, to just blindly accept the faith. And if, under the non-existent chance, Christianity is not true, I don't care if asking those questions leads me to lose my faith, because I want to lose my false beliefs. That's actually why I'm a Christian in the first place.
Another objection I foresee being raised (see, I self-criticise my arguments) is that the verse I cited only says don't ask questions which might disturb you if answered. But this only makes it slightly better, because we're talking about questions that might cause a Muslim to lose his or her faith here. In other words, important questions. With 1 Thessalonians 5:21, there is no limit. I can ask any question I want, even if I think I won't like the answer. Will my atheist dad go to Heaven? I'm not going to like the answer to that question, and if I ask it again when he dies, I'm going to like the answer even less. The answer is disturbing, but I'm not going to lose my faith over it. With Muhammad, there is a limit. In fact, that limit is no questions. It's one thing to say that your religion is so weak that it can't withstand certain questions, like why does Allah seem to "hasten" to fulfil Muhammad's desires (Sahih al-Bukhari, Volume 6, Book 60, Number 311), it's another thing entirely to prescribe death to "hypocrites" (weaker, less devout Muslims).
Can you see Paul sentencing apostates to death? Can you imagine Peter demanding total submission to everything he said regardless of how silly it sounds? Can you name even one time when Jesus refused to answer an honest question? (Note that offering the resurrection as His one sign to hard hearted Pharisees does not count because he wasn't refusing them on the basis that they asked, but that they actually accused Him of being demon possessed, and thus were not worthy to receive such a sign).
Muhammad did not respect the intellectual abilities of Muslims, past or present. He was against questioning Islam, whereas the Holy Spirit, through Paul, told us to test all things and hold on to that which is good. If you're a Muslim reading this, I urge you to ask the very questions Muhammad didn't want you to ask. If Islam is true, and losing your faith is a bad thing, you shouldn't need to fear losing your faith because truth should withstand the test. If Islam is not true, it won't stand the test, but you still shouldn't fear losing your faith because bad faith is not worth holding on to.