Based on the hymns of Gatha, the actual nature of Zoroaster is divine, nevertheless, Avesta’s famous teachings show signs of polytheism, making it an undivine faith. These signs have actually been innovated by later Zoroastrians known as the Manis into the faith. Though it is a religion bearing a divine prophet and heavenly roots, it has strayed from its true teachings that stem from revelation.Islamic traditions from the infallibles confirm its initial divinity, while also confirming that it has undergone major distortion later on.
Theory: “Zoroastrianism is a divine faith”
1- The essence of the its teachings
2- Quranic approval of its divinity
The essence of the teachings of Zoroastrianism:
a) The core and essence of Zoroastrianism
Considering what the Gathas say, the Zoroastrian faith is 100% divine. After doing research on what the Gathas say, most experts in the field of Zoroastrianism concluded that Zoroaster spoke of pure monotheism. Polytheistic myths in Avesta’s teachings appeared in the later Avesta and in the Mani sect.
As a matter of fact, the monotheism Zoroaster propagated was changed by his followers to a polytheistic faith, similar to what happened in Christianity.
We can conclude that the religion of Zoroaster was a divine one at birth, but the current book of Zoroastrians, Avesta, implies polytheism. It is because of this that when we refer to Zoroastrianism, we say it is a distorted faith far from its original teachings.
b) Ideological teachings of Zoroastrianism
God in Zoroastrianism: The Gathas teach that God is one and unique. He is the Creator of the world. A creator who is not affiliated to a certain place, time or tribe. They introduce God as is the absolute knowledge who is the Creator of all phenomena and who is the greatest, merciful, just and powerful. Clearly, such a belief leaves no room for idols or a second hand God. It is interesting that the Gathas see Zoroaster as a Prophet of God and true monotheist bearing, calling Him at the highest levels of monotheism, saying:
“O granter of existence! I humbly have a question to ask you. How is one who is in love with you supposed to worship you? Oh great beloved one! My heart is full of your love, may it be that we are aided by you under in line with correctness and truth and that our hearts are enlightened by your pure light.”
The world in Zoroastrianism: The world is the creation of God and He is its protector and guardian. The world is totally depended on God in a sense that nothing can happen without His will. Ahura Mazda (God) has created the world for ethical purposes.
Man in Zoroastrianism: Man has a high status in Zoroastrianism. Unlike Christianity, it believes in the purity and sinlessness of people at birth. Zoroastrianism says that humans have free will and that it is he who has to choose between good or evil.
Life after death in Zoroastrianism: Just like in other faiths, it says that the soul of man doesn’t die as a result of physical death. Man will go to heaven or hell based on his actions. In the Gathas, there are teachings that differ from those in Avesta. One of the principles of its teachings which are referred to as “the first Zoroastrianism” is that after death, man crosses the bridge of “Cheenood” meaning selection. It’s a bridge that the sinners cannot pass. The fate of the bad will be hellfire, and the fate of the good will be paradise. Occasionally, the Gathas speak of a world after death. Anyhow, crossing the bridge of “Cheenood” perhaps relates the issue of resurrection in Zoroastrianism.
A contemporary Zoroastrian writer believes that man’s eternity after death is the reward of his good deeds, and also believes resurrection to be one of the fundamentals of Zoroastrianism.
Anyway, the bridge of Cheenood can be a reason to attribute believing in a hereafter to Zoroastrianism.
Quranic approval of Zoroastrianism’s divinity:
The holy Quran calls the followers of Zoroaster the “Majus”. Hadiths from the infallibles introduce the Majus as people with a prophet and book. These traditions tell us that the fundamentals of this religion were altered by Zoroaster’s followers. Therefore, if one wants to make the right conclusion from these hadiths, he will have to say that they assert that Zoroastrianism has undergone alteration, but don’t determine what type.
Related question of this website: Materialistic resurrection according to the People of the Book, Question 1916 (website: 1917).
 The Gathas are a collection of hymns that were written 3500 years ago that have illustrated the right way of life through poetry. See: Jalaluddin Ashtiyani, Zartosht, vol. 6.
 See: Din Shenasiye Tatbiqi,pg. 107. Despite this fact, Zoroastrian thinkers try to somehow interpret implicative Zoroastrian teachings with a philosophical and gnostic perspective in order to justify them and prove that their religion is a pure monotheistic one.
 Zartosht (Zoroaster), pg. 122; Din Shenasiye Tatbiqi, pp. 107-108.
 See: Zartosht, pg. 133.
 Din Shenasiye Tatbiqi, pp. 109-110.
 Din Shenasiye Tatbiqi, pg. 110.
 Ibid, pg. 112.
 These teachings have been mentioned in six principles, and this point has been mentioned in the sixth.
 Adyan Asiayi, pp. 42-43; quoted by Abdul-Rahim Sulaymani Ardestani, Seyri dar Adyane Zendeye Jahan, pg. 112.
 Tarikhe Tamaddun, pg. 246; See: Din Shenasiye Tatbiqi, pg. 112.
 Hajj:17 “إِنَّ الَّذِینَ آمَنُوا وَ الَّذِینَ هادُوا وَ الصَّابِئِینَ وَ النَّصارى وَ الْمَجُوسَ وَ الَّذِینَ أَشْرَکُوا إِنَّ اللَّهَ یَفْصِلُ بَیْنَهُمْ یَوْمَ الْقِیامَةِ إِنَّ اللَّهَ عَلى کُلِّ شَیْءٍ شَهِید”.
 Abd Ali Al-Arusi Huwayzi, Nurul-Thaqalain, vol. 3, pg. 475; Hasan Hurr Al-Ameli, Wasa’ilul-Shia, pg. 96; Naser Makarem Shirazi, Tafsir Nemouneh, vol. 14, pg. 46.