Remember that the Roman Empire was at its height when Christianity first came into being, and slowly crumbled therafter. The Romans went from targeting Christians to accepting them to basically ignoring them, as their problems became bigger and bigger.
Yes, the policy of religious accommodation that #3 speaks of was undoubtedly a key factor. However, at the same time, the divine status of the emperor was also something against which Christianity could define itself. The way in which Christianity opposed the Roman religion helped its cause in the sense that it provided discontented peoples that had been taken over by the Romans with a religious system of thought for opposing Roman religion.
One of the major things that contributed to the rise of Christianity was the fact that the Roman religion was not really an exclusive religion. People were allowed to believe in whatever they wanted as long as they also did sacrifices and such to the Roman gods. This created a situation in which various religions, including Christianity, could grow.
Christianity was intermittently persecuted by a number of Roman Emperors, primarily Diocletian and Nero. However, it appeared at a time when interest in the old Greco-Roman gods was in a state of decline. Additionally, Christianity appealled to those who had no status in the empire; primarily women and the poor. Those individuals who embraced Christianity developed a sense of spiritual accomplishment, of a great reward in heaven greater than any earthly reward. Because women were accepted as equals with men, its appeal grew rapidly. By the third century, Christianity was the most popular religion in the Empire. There is some argument that Constantine's conversion to Christianity was not the result of a true conversion, but a rather pragmatic adjustment to appeal to the masses who had already moved in that direction.