From the outset, I wish to make clear that I am not going to discuss Zoroastrianism in any great depth as this is not the authentic and original religion of the Iranian peoples, but a much later imposter and interloper. Instead, I am going to attempt to reconstruct the postulated Sky Father linguistically, and it certainly isn't Ahura Mazda!
The Proto-Indo-Iranian Sky Father is *Dyā́uš Pitā́r, a name inherited directly from the PIE *Dyēus Ph₂tēr, the Vedic Sanskrit reflex being Dyaus Pitar. Thanks to Zoroastrianism, the Iranian reflex was erased from history; the likely old Iranian reflex would have been *Dyāuš Pitā. I have placed an asterisk before the name to indicate that it is a reconstruction, not an historically attested term. As one can see, it is closely related to both the Vedic Sanskrit and Indo-Iranian reflexes as well as the PIE original.
The next logical question is why did the Zoroastrians erase the name and memory of their primary deity, and yet preserved the names and some of the functions of the 'lesser' ones?
Zoroaster (c.624-599 BCE) introduced a system of moral and religious dualism which did not exist in earlier Indo-Iranian religion, whose gods, like those of all the other Indo-Europeans were morally ambiguous (a reflection of mankind). Zoroaster rejected the old gods because he saw them as chaotic, violent, ritually excessive and not aligned with truth and order. Before him deva/daiva simply meant 'god', but he demonised the word and thus demonised the old gods. The daevas thus became demons, and this caused a cultural and religious break between the Indo-Iranian peoples. While the Iranians inverted the meaning of daevas and ahura, the Indo-Aryans retained the original meanings of the terms: deva = god, asura = demon. Those old Iranian gods who were not deemed a threat to Zoroaster's reforms were retained but downgraded from their full divine status to that of a yazata, a divine being, 'one who is worthy of sacrifice/veneration', holy immortals who assist Ahura Mazda in maintaining asha (cosmic order).
Some gods which survived the Zoroastrian reforms:
Mithra He was a far too important and deeply entrenched deity for the Zoroastrians to erase or demonise. He was a guardian of contracts, oaths and light. Under Zoroaster's reforms He became a yazata, albeit a major one.
Anahita She was originally a water and fertility goddess who became Arədvī Sūrā Anāhitā, the holy river and a patron of water, fertility, sovereignty and war.
Tishtrya A rain-bringer and star god who became the divine opponent of drought (Apaosha) and the yazata of Sirius.
Vayu The ancient wind and storm god who survived as Vata-Vayu, a dual god with both benevolent and malevolent qualities in the Zoroastrian tradition.
Rashnu A deity of justice who survived and became the judge at the Chinvat Bridge and the embodiment of truth and justice.
Verethragna A warrior and victory god who became a yazata of victory.
Haoma The Indo-Iranian sacred intoxicant god became the divine embodiment of of the haoma plant.
Major Indo-Iranian gods such as Indra, Saurva and Naonhaithya were demonised, Indra becoming the chief of the demons and named Indra daeva. Thus those old gods whom Zoroaster deemed 'good' survived as yazatas; those whom he deemed violent or ambiguous he simply demonised. We have seen the same process with Christianity demonising popular gods and renaming those who were less of a threat to it as 'saints', the vast majority never having existed as human beings. It is not only the Abrahamic religions of Judaism, Christianity and Islam who demonstrate intolerance towards their rivals, but non-Abrahamic monotheistic religions such as Zoroastrianism.
There is little more that can be said about *Dyāuš Pitā, because all that we have is the linguistic fossil of His name, but I felt it important that this series of essays did not ignore Him. When I approach the Vedic equivalent of this god, Dyaus Pitar we will gain some insight into His Iranian reflex.
