In the case of Christianity, it was by fiat of the Roman Emperor! Roman culture had its panoply of gods and goddesses, and borrowed heavily from Greek examples. In the Graeco-Roman world, religion served to unite the various peoples from the disparate parts of the expanding influence of Rome. However, by about the year 300, the established Roman Empire was breaking down, and cultural differences between inhabitants became more extreme -- barbarians within and without Rome brought their own culture, which included competing pagan philosophies. Attempting to restore some religious unity to the Roman Empire, (since political unity was rapidly disintegrating) Emperor Constantine first recognized Christianity, then made it the official state religion, persecuted those who practiced the old Roman pagan rites, and finally became a Christian himself. Perhaps the concept of a monotheistic male religion in Christianity had less to do with spirituality, and more about maintaining temporal control.