The most striking difference between Zoroaster's doctrine of God and the old religion of India lies in this, that while in the Avesta the evil spirits are called daeva (Modern Persian div), the Aryans of India, in common with the Italians, Celts and Letts, gave the name of deva to their good spirits, the spirits of light.
Unfortunately in a question of this kind the dating of our documents is the first matter of importance, and it seems that we can only assign dates to the different parts of the Avesta by processes of finedrawn conjecture.
These gthas are what they claim to be, and what they are honored in the whole Avesta as beingthe actual productions of the prophet himself or of his time.
The geography of the Avesta points both to the east and the west, particularly the north-west of Iran, but with a decided tendency to gravitate towards the east.
The Iranian tradition, preserved in the Avesta and in Firdousi's Shahnama, localizes a part of its heroes and myths in the east of Iran, and has transformed the old gods who fight with the great snake into kings of Iran who fight with the Turanians.
Only the evil serpent Azhi Dahaka (Azhdahak) is domiciled by the Avesta in Babylon (Bawri) and depicted on the model of Babylonian gods and demons: he is a king in human form with a serpent growing from either shoulder and feeding on the brains of men.
Our Zend-Avesta does not mean the Avesta in the Zend language, but is an incorrect transcription of the original expression Avistgk Va zand, i.e.
According to the Arda-Viraf--Nama the religion revealed through Zoroaster has subsisted in its purity for 300 years, when Iskander Rumi (Alexander the Great) invaded and devastated Iran, and burned the Avesta which, written on cowhides with golden ink, was preserved in the archives at Persepolis.
According to Darmesteter, the Zarathustra of the Avesta is a mere myth, a divinity invested with human attributes, an incarnation of the storm-god, who with his divine word, the thunder, comes and smites the demons.
So, too, fire-worship, especially of the sacrificial flame; the preparation of the intoxicating soma, which fills man with divine strength and uplifts him to the gods; the injunction to good thoughts and good works, imposed on the pious by Veda and Avesta alike: the belief in an unwavering order (rta)a law controlling gods and men and dominating them all; yet with this, a belief in the power of magical formulae (mantra), exclamations and prayers, to whose compulsion not merely demons (the evil spirits of deception druh) but even the gods (daeva) must submit; and, lastly, the institution of a priesthood of fire-kindlers (athravan), who are at once the repositories of all sacral traditions and the mediators in all intercourse between earth and heaven.