Many European apologists of Islam of the nineteenth and twentieth centuries had a far greater knowledge of Islam and were, by contrast, devout Christians - priests, missionaries, curates - who realized that to be consistent they had to accord Islam a large measure of religious equality, to concede religious insight to Muhammad. They recognized that Islam was a sister religion, heavily influenced by Judeo-Christian ideas; and Christianity and Islam stood or fell together. They knew that if they started criticizing the dogmas, doctrines, and absurdities of Islam, their own fantastic structure would start to crumble and would eventually crash around them.
As I've already demonstrated in my post Beware of the Dog, they fear us because we represent reality. But Ibn's point above explains why they would rather have their children marry someone of any other religious affiliation before marrying an atheist. Every theist knows deep down that his specific religious beliefs are just as unsupported as the next guy's, so criticizing someone else's beliefs would be tantamount to criticizing his own. Therefore, even though they may not see eye to eye on everything, most religions (and the various sects and denominations within a particular religion) have an unwritten agreement to show some tolerance toward each other in order to keep them from destroying themselves.
Thus, it is safer for a Christian to marry a Mormon or a Jew or a Muslim before marrying an atheist, because we will most likely call them on their bullshit.