In OD&D, clerics are split between the forces of Law and Chaos. While for some time a cleric can remain undecided between the two, "Clerics of 7th level and greater are either "Law" or "Chaos", and there is a sharp distinction between them" (M&M p. 7).
While the clerics of OD&D were most likely meant to represent medieval orders of Christian knights (Gary himself was a practicing Christian and often inspired by his faith), I find Zoroastrianism a much better fit.
Zoroastrianism was [is] a dualistic religion, meaning that it was neither mono or polytheistic. There were two gods: Ahriman and Ahura-Mazda. Ahriman was the god of darkness while Ahura-Mazda the god of light. According to Zoroastrianism, these two beings are engaged in an epic cosmic conflict over the universe that is thought to last 12,000 (about 6,000 years from the present day).
Some of the minor, but defining features of Zoroastrianism beliefs are demons that were once gods, the resurrection of Ahura-Mazda, angels (of both Ahriman and Ahura-Mazda), and fire alters and the centers of religious worship, all of which fit quite well into the OD&D framework.
If you had to sum up Zoroastrian ethics with a single line from the Zend-Auestra (the sacred Zoroastrian text), that line would be "good thoughts; good works; good deeps."
What really strikes me is the cosmic battle between light and darkness, or, in OD&D terms, Law and Chaos. I also find that reverse spells fit very will into this paradigm.
