"Doctrinal fidelity in the [Zoroastrian] cult of Mithra can thus be demonstrated over a period of at least 2,500 years.
"Close doctrinal fidelity by the Zoroastrian church can be established in other respects also; and the veneration in which it holds its prophet is shown in many ways. Yet by the syncretic theory one is asked to believe that profound respect for Zoroaster, and a proven tradition of immense conservatism and loyalty, can both be reconciled with an early, radical betrayal of Zoroaster's own teachings; and that in the case of Mithra, the prophet's disciples, although scrupulously preserving his own words and his moral teachings, so far rejected his doctrines that they put their worship of the god whom he preached, Ahura Mazda, under the protection of a god whom he denied, or even abhorred[, namely, Mithra]. To establish the syncretic theory against such opposing considerations would require very strong evidence indeed; and in fact, as we have seen, there is no real evidence for it at all. It is reasonable, therefore, to reject it, and to accept instead the testimony of the Zoroastrian church, unchanged and harmonious at all known periods of its history. From it one can deduce that Zoroaster held to the basic theology of the old Iranian religion, with all its yazatas[, including Mithra], and that his reform consisted largely in reinterpreting its beliefs at a nobler and subtler spiritual level, in the light of an intensely personal apprehension of the supreme God, and of the struggle to be waged between good and evil.
"The immense help given over the last century and more by comparative philology for the better understanding of the Avesta, and the great advances made, have led perhaps to a touch of hubris in the West, to an assumption that on all points juddīns can interpret the Good Religion better than its own adherents; but this is a sweeping assumption, and the study of other religions suggests that it is unlikely to be true. Plainly there have been considerable theological developments in the course of the long history of Zoroastrianism; but there is little sign of those radical breaks and changes in doctrine which have been so widely postulated by Western scholars."
Mary Boyce, "On Mithra's part in Zoroastrianism," Bulletin of the School of Oriental and African Studies, University of London 32, no. 1 (1969): 34 (10-34). Background: The "particular Iranian doctrine which is generally held in the West to have been rejected by Zoroaster" is "the doctrine that Mithra existed, that he was a great and good god, and that he was to be worshipped. Most Western scholars have held that Zoroaster denied the existence of Mithra, or was vehemently opposed to his cult, or tacitly ignored it" (14). But this was due to the above-mentioned "touch of [Western] hubris". Moreover, "the present Zoroastrian veneration of Mihr, as protector under the Creator Ōhrmazd, is wholly consonant with what is regarded as the oldest allusion to Mithra in the Avesta" (33).
If Boyce is right, and Zoroaster did not break with "the old Iranian religion" over Mithra (nor has Zoroastrianism ever done), that is nevertheless not the same thing as (for example) the apparently highly dubious claim that the Mithraism of the 1st through 4th/5th-century Greco-Roman West was a form of Zoroastrianism plain and simple!
Something else worth excerpting: "The Zoroastrian tradition is firm that the Zoroastrian church is one, and that it was founded by Zoroaster, who was a great prophet but a mortal man, living at a particular time in history. All the marvellous legends of his birth and life have not obscured this basic tenet. The day of Zoroaster's death is remembered each year in Iran and India, on Rūz Khoršēd, Māh Dai, and a bāj (i.e. the drōn ceremony) is then solemnized in his honour. This service is celebrated for him as for a righteous man who has died, an ašo ravān; and since no Zoroastrian act of worship may be offered to a human being, however holy, the service is celebrated with the xšnūman of Ardā Fravaš, but with a special intention (nāmčišti) for the soul of Zoroaster. . . . This liturgical fact is of primary importance as evidence for Zoroaster’s human existence. If his own followers have resisted the pious temptation to make their prophet divine, there seems little justification for juddīns to do so” (12).