Well, this is just a though experiment, but I don't think it's entirely pointless. Other so-called prophets would rise and fall. Does one of them essentially take Jesus's place? Would his followers turn him into the Messiah, as it has been accused of Jesus's followers? Would what follows approximate Christianity as we know it, or would it never gain traction?
Well, it probably wouldn't gain traction, but what I mean is that if there was no "the Christ", it wouldn't be "Christ-ianity", and it follows that, since the apostles were followers of Christ, if there were no Christ, the apostles wouldn't have met or associated together, hence no nucleus for a new religion to form around. For that matter, without the
koine of Latin or Greek, the rabbi and carpenter Jesus of Nazareth would have been a local folklore figure at best, since his teachings simply wouldn't have spread through cultural barriers - a unified trade language let word of his lessons and actions spread throughout the region. Actually, without the persecution by the Romans supplying the oppressive authority, he would have been a disruptive teacher among others and very possibly been forgotten in time, except as a footnote in some religious scholarly records. No crucifixion means no martyrdom and resurrection, meaning no Christianity.
Islam probably wouldn't be all that different as a system of belief, since it is derived from the Old Testament. The following conflict between Western Christianity and Middle Eastern Islam, however, might be very different. If the Romans had never adopted Christianity, the Germanic Tribes would never have converted, and so Europe would be an absolute mess of pantheons with purely local concerns. The unifying force of Islam might very well have found fertile ground for their religion there.