Mono-Mono

Question: I once heard that Zoroastrianism is a monotheistic religion, but then you have Mithra and the other gods. What's going on? Was I just misinformed?

You kind of were, but it's a common piece of misinformation. First, let's talk about what monotheism is, and then we'll dive into how it affects Zoroastrianism!

Zoroastrianism is often called a monotheistic religion because it holds Ahura Mazda, the omnipotent and all-wise, as the most powerful being in the universe, the eventual inevitable defeater of evil, the creator of all things and the sole source of benevolence in the universe. He's very comparable to the big-G Gods of Judaism, Christianity and Islam in power, scope and universal benevolence, and has probably become even more like them over time as those religions (especially Islam) grew up beside and influenced Zoroastrianism.

But Zoroastrianism isn't actually monotheistic. It's monolatristic, meaning that it acknowledges that other gods exist, but worships only one of them as the supreme power in the universe, whereas monotheism believes that there is only one god and all others are either nonexistent or some lesser power pretending to the title. Zoroastrianism is full of references to other gods, from the daevas being acknowledged as gods who are simply evil by virtue of being misled or purveyors of lies that come from Angra Mainyu, to the presence of the Yazata who are expressly acknowledged as divinities, to the references to Ahura Mazda as greatest among deities, not simply the only one that exists.

(In case you were wondering about the difference, another closely related concept is that of henotheism, which is the practice of worshiping a particular totem god above others despite acknowledging others that are equally worthy of power. The quick rule of thumb to tell the three apart is that monotheism says there is only one god, monolatrism says there is only one god worth worshiping above the others, and henotheism says there are lots of worthy gods but only this one is worshiped by a particular people.)

Zoroastrianism does have a lot of concepts in common with monotheistic religions; it has a strong good/evil duality, a single all-powerful god-figure that rules the universe and a heavy focus on the purity and good works of its worshipers to guarantee their passage through judgment day and into a heavenly afterlife. That, and the strong influence of monotheism in Persia and nearby areas, have combined to make casual readers about the religion often lump it in under the "monotheistic" label in error.

Actually, some of the Yazata, most notably Mithra and Anahita, are likely to predate Ahura Mazda completely; there's some evidence (usually from neighboring cultures that recorded Persian culture before they were recording it themselves) that they were worshiped in ancient Persia before the rise of Zoroastrianism, which only arrived on the scene around 600 B.C.E. There was certainly an ancient set of polytheistic Persian gods that were worshiped prior to Zoroaster rolling in and setting up Ahura Mazda as the supreme deity of the region, and several of the Yazata were probably among them, then translating on into the new religion as integral parts of it.