Before coming to Yahoo chat rooms more than a decade ago I knew little about Zoroastrian religion. I met some very nice practitioners of their faith in a user chat room and they were some of the nicest people I encountered in Yahoo chat. It was from practicing Zoroastrians that I learned that their religion does not fit the definition of literal monotheism.Monotheism means belief that only one God is real. Belief there is only ONE not 2 or 3 nor in dualities, trinities, nor in in demi gods, nor in incarnate deities. NO other entity with the power or attributes of deity, nor any other entity worshipped. The Zoroastrian religion isn't monotheistic as they have more than one deity. Angra Mainyu is the destructive deity, Ahura Mazda the creative deity;it has a concept of dualities of deities and in some groups, Zoroaster himself became an object of worship.
Judaism is the ONLY true monotheistic religion of those brought up because UNLIKE Christianity and Islam who have a concept of a Devil that "rules" the "underworld" and has the power to BATTLE God thus giving this entity the attributes of a deity to be capable to battle God. In Torah & Judaism there is NO other entity that can battle/threaten or challenge the power of God. HaSatan in the Tanakh is an angel of God who needs God's permission to act and is never shown disobeying God. HaSatan is the adversary to humans, not God in the Torah. He represents the inclination to evil that humans can master. Humans are more powerful than the satan in Jewish belief if we reject the evil inclination. The commandment to have no other gods before me acknowledged that **other people** worshiped what they believe to be deities. Torah did not acknowledge those other entities as having any divine power, and repeatedly showed the impotence of false gods.Torah's concept of the nature of God is an incorporeal omnipotent Creator that does not incarnate. The notion of a human becoming a god is condemned in several lengthy narratives in the Tanakh. The *attributes* of the nature of God are many, including merciful and just, not simply masculine and feminine, but in no way are they corporeal or separate manifestations. God is strictly monotheistic in the Torah. Thus, the trinity concept of Christianity isn't compatible with strict monotheism either as it possesses a concept of an incarnate, corporeal manifestation of a god in human form. The worship of or through Jesus as a god incarnate renders Christianity to be incompatible with strict monotheism, too.
With regard to why Ahkenaten's worship of "Aten" also fails to be monotheism. Ahkenaten's worship of "Aten" (light beyond the sun) represented by the disc of the sun, was still a far cry from the monotheism of Torah primarily because Akhenaten was still, like the Pharoahs before and after him, believed to represent the deity on earth as an incarnate manifestation of deity. Objective physical archaeological evidence to the belief of Akhenaten as Aten incarnate is found inscribed upon one of this thrones. The monotheism of the Torah is about a covenant of faith that is more concerned with this life rather than the obsession of the Egyptians with the afterlife. The book of Exodus reads as a direct replacement theology, treating the belief of Ahkenaten in a somewhat similar manner that the New Testament replcement theology does to Judaism! Those who do not recognize that similar terms have taken on topsy turvy meanings in the context of the doctrine, think they're far closer than reality. There have been all sorts of theories that Moses may have been influenced by remnants of the old Aten faith of Akhenaten. I think it more likely Ahkenaten was influence by those Hebrews who had not become apostate to exclusive worship of God while in Egypt for generations since coming there.If the Pharaoh at the time of the Exodus was Ramses (as generally believed) then Nefertari as Rameses’ primary wife, believed to have been the niece or grandniece of Queen Nefertiti, wife of Akhenaten, could have taught the young Moses some of the philosophy of the banned religion and it could have influenced Moses to some degree, just as other cultural influences are also found within Judaism. However, the influence was toward rejecting the beliefs in any corporeal deity.The more I studied the religions of ancient Egypt, the more I recognized that the Torah was the antithesis of the Egyptian religions, even in how Jews have always referred to our eternal covenant law, Torah as the Book of Life, versus their Book of the Dead.
BECAUSE Zoroastrains do not have only one god in their religion that they are not a monotheistic religion. They may have decided that one was a supreme deity, but as the Hindu religion may have a supreme deity over others, the fact they have other deities means they're not monotheistic
edit: The Zoroastrians I've met did believe themselves to be monotheistic, just as Catholics who have a Devil who *rules hell and can battle God* and who believe *Jesus is god in human form* believe they are monotheistic. Zoroastrians DO have at least 2 deities and some of them even invoked Zoroaster himself to direct their prayers. http://www.pantheon.org/articles/a/angra...