Islam is a bizarre syncretist cult. It was built of bits and pieces of the religions existing in the area at the time, but of course carefully structured to promote Mo and his descendants, who have been living well off it ever since. Mohammed was a pagan, many Arabs were Christians (although the Arian heresy had confused their Christianity and separated them from the Church in many areas), and there were also many Jews living there. Hence his "visions" conveniently combined the three.
Sometimes the bits and pieces don't fit together very well, but Islam is so anti-intellectual that few Muslims know and fewer care and the ones who do question it usually get killed.
Mohammed was basically a bandit chieftain. Islam is a religion designed for ruling a bunch of bandits.
You're definately right about many of the pre-Islamic Arabs being Christian--many of whom were of cults like Arian that, when Islam rose, seemed to sort of just blend into Islam.
But the point about anti-intellectual is wrong in someways, because the Islam of today is not the Islam of the religion's "golden age." They seem to have gotten stuck on the "good old days" syndrome and many want the world to revert back to that stage. Islamic scholars are responsible for the number 0, as well as establishing the first hospitals for the mentally ill (in Baghdad of all places), and being far ahead (at the times of the Crusdades, at least) in understanding medicine...
There is an interesting story I read about, when during the Crusades a Christian doctor who had grew up in the Islamic society, tried to treat a Christian soldier wounded somewhat badly in the leg, and then a woman inflicted with "consumption." He cleaned and put anti-bacterial balm on the leg and simply recommended that the woman change her diet.
Then a doctor from France, who had traveled with the Crusaders, came in and told the soldier that he had to lose the leg or his life, and instruced his assistants to drill holes in the woman's head because she was over wrought by demons. Needless to say, both the patients died, and the Arab-raised Christian doctor swore off interacting with Crusaders.