Why were Roman emperors threatened by the spread of Christianity

Because Christianity was so diffuse, and also illegal and therefore kept underground, it is hard to speak of a united "Christianity" in this period. A more accurate statement might be that there were "Christianities," that is, various forms of Christianity, with different takes on the religion and its message. Gnostics, Montanists, Docetists, and others, debated about the true nature of Christ, salvation, and the life of a Christian.