Originally Posted by troung

The Arab invaders of the Middle East avoided forced conversion of the entire population for numerous reasons - the small size of their own forces (some historians put it at some 20-30 thousand), leaving well enough alone and of course the most important, money. Taxes mixed with the somewhat simpler one god issue (ever wonder why they say there is no god but god so much - its a slam at the trinity) got it spread in the Middle East after the conquest. And the Ummayads problem with converts got them overthrown. There was initially an issue over converting anyone. Indonesia and Malaysia picked it up off of trade. And heads were cut in India. And of course there are still Jews, Christians and Zoroastrians in the Middle East. Treatment of religious groups differed from dynasty to dynasty but if they all had some official policy to make them all Muslims by the sword then there would not have been a single non-Muslim there today. So the spread was mixed.

And how did Christianity spread? You will find plenty of headless corpses as well as people who thought Christ was the man.