Zaruthstra and Mithras: The History of the Ancient Persians' Religious Influence on the Roman Empire

The Sasanian Empire was important for a number of reasons. Besides being the last of three great Persian dynasties, they carried on many Persian cultural traditions relating to religion and kingship. The Sasanians fostered and promoted the native religion of Zoroastrianism to the point of persecuting other religions from time to time. It was during the Sasanian period that the numerous Zoroastrian hymns, prayers, and rituals were collected under one book, known as the Avesta. Thanks to the Sasanians' efforts with regard to religion, modern scholars know much more about Zoroastrianism than they would have if the religion continued to disseminate orally. Their efforts also protected Zoroastrian knowledge in later years after the dynasty was long gone and Islam became ascendant in Persia.
In the early years of the Roman Empire, as Christianity struggled to gain a foothold and survive in the polytheistic pool of Roman theology, its greatest rivals weren't the Caesars or the Roman aristocracy but rather the faith and devotion of the common Roman legionaries. The faith of these men was centered on the god Mithras, who, they believed, led them to victory upon the field of battle and had done so for nearly four centuries.
Despite this widespread belief among soldiers, the cult of Mithras was not a creation of the Romans, although they would eventually add their own rituals and mysteries to the ancient religion. In fact, the Mithraic religion was an Indo-Persian creation, a theology which managed to travel from India and back into the Hellenic and Roman world by way of Alexander the Great's conquest of the Persian Empire. Of course, if the Mithras cult began as a Roman interpretation of some elements of Zoroastrian theology, then new questions are raised. Perhaps the most obvious question is why the Romans would adopt one of their biggest enemies' gods.