In 1912, James Hope Moulton presented a series of lectures in Oxford which were later published in the Hibbert Lectures (1913). His topic was "early" Zoroastrianism by which he means Zoroastrianism, its immediate predecessor beliefs, and its descendant beliefs up to the time of Alexander. The work is quite valuable, but uneven. Moulton's first two chapters, "The Sources" and "Before Zarathustra" are somewhat unclear and confusing. His frequent name dropping and brief allusions, indicates that he was addressing an audience that was well-informed about the prevailing issues in Zoroastrian scholarship. After almost 100 years, it's all a bit baffling to the casual reader.
In the first chapter, Moulton presents numerous arguments for various theories about when Zarathustra lived. His evidence does not provide a clear and simple conclusion, but in general it tends toward an earlier date than what is often suggested. The clearest evidence for Zoroastrianism is the Behistan bas relief and inscription depicting Darius around 500 B.C. It is often thought the Zarathustra must have lived about a century or two before this; however, the arguments Moulton relates could place Zarathustra three or four hundred years early at the start of the first millennium.
Moulton's third chapter, "The Prophet and the Reform" is clearer. Here, Moulton argues that Zarathustra flourished in Bactria and traces the development of his thought -- portrayed as a great reformation of early Aryan nature worship -- though its transmission west by the Magi. In the hands of the Magi, Zarathustra's religious insights were transformed and -- in many of their essentials -- lost. The course of this transformation is described in two chapters, "The Magi" and "The Magi (continued)." Moulton argues that many of the more popular (and sometimes disturbing) notions of Zoroastrianism are actually alien to Zarathustra's teaching and were grafted onto the religion by Magi who, having failed to maintain political power in the Achaemenian period, established themselves as religious leaders.
The rest of the work addresses more specific features of Zoroastrianism and happily is clearer and more direct. The polytheistic doctrine of the divine beings (amesha spenta) is presented as a departure from Zarathustra's monotheism. Similarly, the moral dualism setting the good god Ahura Mazda against the evil Angra Mainyu (or Ahriman) is presented as a Magian perversion. This has come to be known as the Zurvanian heresy and now widely accepted.
Zarathustra made, however, a number of very important theological contributions that are prominent in many religions today, including monotheism, the immortality of the individual soul, a future day of judgement when our moral conduct will determine our eternal fate, and the existence of fravashis, or guardian spirits associated with each person. This notion, however, was not so exclusively Zarathustra's, but it was an important component of his thinking and may have been the origin of contemporary Western angelology.
In the final chapter, "Zarathustra and Israel," Moulton takes up a question that occupied the scholars of his time: what was the relationship between the theology of Zarathustra and the theology of the Bible, which have a number of striking similarities. Moulton's chapter is an interesting, though brief argument that while there might have been some cross-fertilization, the two theologies arose independently. For a more thorough treatment of this question, I recommend Lawrence Mills's series of lectures Our Own Religion in Ancient Persia. Mills's lectures also were published in 1913, though their publication was based on work done in past years. It is likely that Moulton had access to at least some of these before writing his own lectures.
Mills outlines three possibilities: (1) Zoroastrianism and Yahwehism arose largely independently, (2) Yahwehism impressed upon Zoroastrianism its monotheistic tendencies when the Jews were held captive in Babylon, and (3) the converse: Zoroastrianism impressed upon Yahwehism its monotheism during the captivity. Mills, like Moulton, opts for the first possibility, but it seems as though both are motivated by a desire either to glorify monotheism by claiming that its truth was independently evident or by the desire to defend the independence of Yahwehism.
If Moulton is correct, that Zarathustra's monotheism along with his other theological insights arose around the tenth century B.C., and if recent archeology and scholarship is correct, placing the origins of the Bible at or around the time of the exile, then it would seem natural that Zoroastrianism was the progenitor of monotheistic Yahwehism. While not absolutely certain, it seems more likely that a relatively minor ethic group (the Jews) might, while in captivity, adopt the elements of the theology of their surrounding culture. This is particularly likely in that prior to the rise of monotheistic Yahwehism in the post-exilic period, the Jews were at most henotheistic and so open to a variety of theologies. Second, their Babylonian captors were far and away the most powerful state in the region. Finally, their Achaemenian liberators (again the most advanced state of the region) were unquestionably monotheistic. It would have been a very easy step to adopt the monotheism of Ahura Mazda under the old, familiar name "Yahweh."
Moulton's Early Zoroastrianism, while somewhat frustrating at the start, develops into an extremely interesting examination of major theological ideas, whose origins are not well known today. It is no wonder that it remains in print today.