Well, first of all, the Greeks had a polytheistic (pagan) society, which the Christians didn't have. They instead practiced monotheism. The Greeks never actually banned the practice of polytheism, but the Christians did. While the Christians were in the process of recruiting gentiles to Christianity, many people were still pagans.The Christians slowly recruited more and more people and there were less and less pagans in the world. Once Christianity became the religion of most societies, anyone, and I mean anyone, who was not Christian, was targeted, not only pagans. In a Christian society, if you weren't Christian, you were treated badly, and that meant the banning of polytheism. Now, there are modern pagans, but had they lived when Christians controlled most societies (they still technically do, but not in the same way they used to), they would most likely have been executed or forced to convert.
Christian religion control
More info on polytheism ban/end of Greco-Roman polytheism