Then there probably would be no Jeudaism (at least as we know it) and less or no Xianity and possibly no Islam.What if Constantine hadn't had that pivotal dream?
While there's the story that Mohammad brought monotheism to polytheistic Arabs the truth is there are many many monotheistic writings concerning Allah well before Mohammad was born. If he existed then he stepped into a culture teetering on the brink of becoming monotheistic anyway. There was already a blending of Arab myths and Xian/Jewish myths. Think of the Mormons. It's just happens.
It should also be noted that the line between Xian and Jew was blurry. Things weren't as defined as they are now. So the first Muslims were more then likely Arab Jewish-Xians, which may seem odd now but was normal back then.
Anyway,
Michael