I am referring specifically to the Roman ruled lands of Egypt, Syria, Lebanon, Israel, and basically everything between the Tigris and Mediterranean. Did Christianity dominate as the majority religion in the middle east among the common populace or did the majority of the population follow another faith before the mass conversion to Islam?(I know not 100% converted, but it did eventually become the majority)
Yes, Christianity dominated as the majority religion across these territories. The population centres across the whole empire were weighted towards the Eastern half, and the strength and depth of the Christian church was in the Eastern Empire, even more so than the West. I would write more, but this is such an established fact that I am tempted to simply state "It is known". In the Roman Empire from 300 onwards, not merely including, but I would say especially, in the East, Christianity eclipsed the practise of Hellenistic polytheisms and dominated the religious socioscape.