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Perhaps the biggest impact empires facilitated is they allowed people from many different ethnic and religious backgrounds to more freely intermix (Figure 1). Ideas now began to be shared among many people, where even polytheistic faiths began to share similar concepts. After the 6th and 5th century BCE, universal ideals became more evident, and this could have been a key period in the formulation of universal philosophies. What is telling is universal philosophies first developed in limited regions around the Near East, where Iranian Zoroastrianism may have played an important, influential role, but this is not entirely clear. What is clear is those universal philosophies were geographically limited in spread until after the 3rd century BCE.
After Alexander had reached India, even Buddhism became influenced by the rising tide of universal philosophies. Zoroastrianism became an important universal faith that shared some common ideas as other universal religions, including Christianity. With the rise of Christianity in Rome, particularly in the early 4th century CE, the Persian Sasanian Empire, Rome's great rival, began to be associated with Zoroastrianism, and Christianity became Rome's supported religion. To create loyalty and obedience, empires began to increasingly support one religion rather than allow many gods and religions. Persecutions soon became common in the major universal states of Rome and the Sasanian Persians. This continued in subsequent periods when the Byzantine Empire succeeded, although the Iranian Sasanians were far more tolerant than Rome and Byzantium had been. Universal philosophies also became a type of political football. The rhetoric of invasions became justified by the often one god for the new universal faiths.<ref>For more on how universal philosophies and religions spread, see: Schott, J. M. (2008). <i>Christianity, empire, and the making of religion in late antiquity.</i> Philadelphia: University of Pennsylvania Press.</ref>
The immediate effect of many of the universal religions is they led to the downfall of many of the old religions that once spanned Eurasia. Gradually, either through persecution, financial incentive, coercion, or just normal belief, most polytheistic religions ceased to exist, outside of some lingering stories and traditions that often became incorporated into the new universal religions. For example, many saints' days were often days in celebration of ancient gods.<ref>For more on polytheistic faiths and the types of worlds they created, see: Johnston, S. I. (Ed.). (, 2004). <i>Religions of the ancient world: a guide.</i> Cambridge, Mass: Belknap Press of Harvard University Press.</ref>