Spencer argues that Islam, unlike Christianity, has a specific theological mandate to expand by force and to convert, kill or subjugate nonbelievers. To this I can only say that, mandate or no, historically Christianity (until relatively recently) does not seem to have been far behind Islam when it comes to forcible conversion, slaughter or subjugation. Christianity has modernized; Islam, by and large, has not. The theological and cultural causes of this can be debated ad infinitum. Islamic reformation may well be more difficult than Christian reformation. It does not follow that it's impossible.Perhaps possible, but unlikely. Forcible conversion, slaughter, and subjugation have been central tenets of Islam for over 1000 years. They have never been central tenets of Christianity. Islam is not just a few years behind Christianity; it is a religion with an entirely different purpose and it is not making progress.
Someone is probably going to send me some obscure story about how some overzealous Christian missionaries tortured some poor natives. Such incidents may have happened, but they are usually grossly exaggerated. Usually I find that the missionaries built schools and hospitals, and I cannot verify the torture.