Right, so I’ve decided to address this on here too.
Our dualism is not a dualism of two opposing but valid “gods” - one “good” and the other “bad” - that govern over their respective but equal domains and who both play equal roles in the creation. NO.
Evil is NOT eternal, nor is evil created by Ahura Mazda.
A lot of people make the mistake in thinking this is some Yin and Yang-type dualism where Ohrmazd and Ahriman are in some neverending cosmic battle and represent a form of “balance” in the Universe. Again, NO.
The only balance, harmony, and order that exists is that of Asha, or Truth... The very antithesis to Druj, Deceit/Lie, which is disharmonious and disordered.
Ahriman and the divs/daevas are more like parasites, they seek only to CORRUPT the inherently good Creation; to take its light and taint it.
Yes, we believe firmly that our world is inherently good, thus we seek to cultivate and cherish it’s goodness and protect it from darkness by facing evil head on and destroying it.
Evil is not valid. Nothing was evil in the beginning! When certain expressions of minoo (spirit, consciousness) decide to not be in tune with Reality, then the spirit/mentality refuses to progress and evolve and to be at one with Asha and thus acts opposite of it.
If we look at the Gathas specifically, “Angra Mainyu” (degenerative spirit) and “Spenta Mainyu” (progressive spirit) are written to be “twins” that are always opposed (antithesis to eachother). This doesn’t mean biological twins (i.e both made by Mazda), but rather the direction and expression they both take are equal - yet contrary - to another. However, one is clearly more potent; the one of virtue, and it always triumphs. Evil is that which purposefully goes against The Good which has always been, and will always be. So this world does NOT need Evil to exist. Goodness is independent of Evil, and does not need it. Yet, it will certainly stand it’s ground should it have to.
Goodness ALWAYS triumphs even if there are malign energies that act contrary to it.
Ultimately, all evil shall be destroyed during Frashokereti, the final renewal of the Universe. We too can destroy it during our own lives.
Originally, everything was made perfect. Evil came into the world/developed later, and it sought only to destroy/cause decay. Like a poison, in a way. Again, corruptive.
Our dualism is more of an ethical dualism, in that yes, there is right and wrong and you have the absolute free-will to choose. One, however, is the truth and is much more powerful compared to the other which shall only do harm. It’s awareness/wisdom vs ignorance, one that chooses life and the other that is anti-life, and the soul/urvan/ruvan can make its own decisions ultimately.
But... Evil does not come from God!
In later writings like the Bundahishn (Middle Persian text concerning our cosmology, non-scriptural text) Ahriman resides in a dark abyss, and Ohrmazd of the superior realm of lights.
Both are limited and unlimited spirits, for the supreme is that which they call endless light, and the abyss that which is endlessly dark, so that between them is a void, and one is not connected with the other; and, again, both spirits are limited as to their own selves. And, secondly, on account of the omniscience of Ohrmazd, both things are in the creation of Ohrmazd, the finite and the infinite; for this they know is that which is in the covenant of both spirits. And, again, the complete sovereignty of the creatures of Ohrmazd is in the future existence, and that also is unlimited for ever and everlasting, and the creatures of Ahriman will perish at the time when the future existence occurs, and that also is eternity.
-> Ohrmazd is omniscient and had already known Ahriman exists. Ahriman, in his backwardness, did not know of Ohrmazd and becomes jealous and upset upon coming to realization of God’s wonder. As well, Ohrmazd understood well what Ahriman’s intent was (to corrupt and negatively entangle/afflict), but out of the Goodness of his nature could not find the will to punish Ahriman (he offers him repeatedly a truce). Thus he fashions the World, as perfect as it’s Creator, to eventually serve as a sort of battle-ground between the two. In addition, the two were un-manifest, they weren’t manifest, they resided within Zruvan Akarana (boundless time/space). In order to defeat Ahriman, Ohrmazd had to fashion the material world w/ finite time so he and Ahriman could become “manifest” (from intangibility) in order for Ohrmazd’s vision (aka causing his evil-twin’s destruction) to actually come to fruition.
On the evildoing of Ahriman and the demons it says in revelation, that the evil which the evil spirit has produced for the creation of Ohrmazd it is possible to tell by this winter; and his body is that of a lizard whose place is filth. He does not think, nor speak, nor act for the welfare of the creatures of Ohrmazd; and his business is unmercifulness and the destruction of this welfare, so that the creatures which Ohrmazd shall increase he will destroy; and his eyesight does not refrain from doing the creatures harm. As it says that, “ever since a creature was created by us, I, who am Ohrmazd, have not rested at ease, on account of providing protection for my own creatures; and likewise not even he, the evil spirit, on account of contriving evil for the creatures.
At first, there was unmanifested Light, and below there was Dark. One did not create the other. One creates the world (as a material trap) and the other enters it, becomes stuck, and is doomed to stay there, albeit the entire time it attempts to negatively afflict that which was already Divine (which it had sought to do when it first invaded it).
Eventually humans are caught in the midst of this cosmic battle, for ultimately it was they who offered to Ohrmazd that they would go and fight Evil face-to-face. Thus humans are God’s agents against the forces of malicious drive. They must repell and eliminate it via good thoughts, words, and deeds, and protecting and cherishing the goodness that already exists in the world (fighting back against any corruption).
This is still like an “ethical dualism”. Repeatedly it is mentioned that the negative power is inferior and doomed from the start, compared to the positive, auspicious force which can overcome anything. These are expressions of consciousness, stagnation/narrowed-mindedness and abundant splendour.
Quite similar to the older religious texts it proceeds from, we assume Ahriman/Angra Mainyu has existed prior. There is no mention that “the evil spirit” is “born” from any entity or source, but rather, chooses to direct themselves the way they do and refuse to progress/evolve thus stay in their limited expression i.e evil. Everything is an expression of Minoo (spiritual energy, consciousness). It can choose Asha or it can choose to become diseased (druj).
In fact, Ahura Mazda offers the evil spirit to redeem and allign with Asha (cosmic flow, order). The offer is denied.
Evil then, seems to be a choice. A choice that dictates nature, which our choices often do.