If Jesus just died off without really gathering any significant number of followers or supposedly rising from the grave, odds are regular Jews in Rome, who also would have been a significant minority in the city due to the Jewish Exodus from Judaea (the Roman Province) would have been persecuted by Nero instead. What let christianity spread like wildfire throughout the city was how the christians were so devoted to their faith. Even while being burned alive as human torches told they were only allowed to be taken down if they renounced their faith, they didn't. This inspired many people throughout the city and empire to convert. Odds are a similar occurrence would happen if Jews were the people who were persecuted in such a horrific manner. Judaism wouldn't have spread as far and wide though, due to the fact that Christianity is designed to get more converts. Judaism is not designed to get converts, so odds are at most 15%-25% of the empire would convert, compared to the 90%-95% of the empire that Christianity took hold of. With no Christianity, Islam never would have developed, due to Jesus and hell being very big parts of the religion, and Judaism not having a mention of either of them. So there you have it.