On Mithra's part in Zoroastrianism

One of the great problems in the study of Zoroastrianism has been to determine what were the new elements in Zoroaster's teachings—elements which roused such bitter hostility among his own people that he was driven to seek exile and to find his first converts among strangers. It is a problem because, through comparison with the Vedic religion, it has been possible to establish a good deal about the religion of the Iranians before Zoroaster, and naturally even more is known about the religion of the Zoroastrian church after him; and the two are remarkably and disconcertingly similar, as if the second were in many respects a natural development of the first, without any break in continuity, whereas neither is very fully or clearly reflected in Zoroaster's own highly complex Gāthās . Western scholars have fairly generally sought to explain this apparent anomaly by postulating that in many points of doctrine the religion of the Zoroastrian church is not in fact that preached by its prophet; they suppose, that is, that the followers of Zoroaster fairly rapidly betrayed his teachings and evolved a syncretic religion in which they reverted to many ancient beliefs and observances which he himself had denounced.