Zoroaster’s message is preserved in the Gathas, a body of 225 verses dispersed over 17 Yasnas composed in an ancient Iranian language.
What made Zoroaster's ideas radical was firstly his revelation that there was one creator, Ahura Mazda, the Wise Lord, at a time when it was commonplace to worship the numerous natural elements as gods in their own right.
His understanding of life was based on his realisation that all the manifestations of creation had to come ultimately from one all-powerful energy - God or the Self-Creator.
His originality is further seen in his injunction found in the Gathas, that those who are listening should use their free will to choose their own path, that of good or that of evil.
In this injunction are two fundamental ideas: free will and individual responsibility for one's own actions; and the concepts of good and evil.
Good and evil are understood as realities encountered in the inner mind - the conscience - that appear to operate as twin energies, equally present and both exerting a pull over us.
Zoroaster's message is basically that we should be aware of the struggle which these two forces engage us in and know the consequences of following one rather than the other.
We are told that to follow the path of "Asha", (good order, harmony, truth, summarized by purity in thought, word and deed) will lead to happiness ("ushta") for both ourselves and others.
The alternative choice of deceit, lies and unkindness, namely, impurity of thought, word and deed, will lead to unhappiness, enmity and war.
Thus Zoroastrians are engaged in an internalized struggle of ethical dualism.
Prior to the philosophy of Zoroaster, entities such as Mehr, Hom, Verahram, parallels of which were found in Vedic texts were venerated.
Although there is no mention of these in the text attributed to Zoroaster, it is believed that this ancient tradition survived and merged with Zoroastrianism so that they re-acquire their earlier prominent positions in the religious recitations by priests and lay people.
A reflection of this is found in the name of Dar (b)e Mehr which in Farsi denotes the temple.
Zoroastrianism has had an impact that is rarely realised in the Judaeo-Christian world through its contribution to the concepts of heaven and hell deriving from its ethical dualism.
Associated with this was the concept of the Day of Judgement, at which point, on the third day/fourth night after death, the soul crossed a bridge, Pol e Chinvat, on which its good deeds were weighed against its bad deeds.
The outcome of this balance determined whether one would pass through to the abode of eternal light and happiness/the house of everlasting song or be plunged off the bridge into an eternal limbo.
These concepts developed gradually over one thousand years after the teachings of Zoroaster, and not surprisingly, they underwent interpretations and consequent distortions.
Ohrmazd (Ahura Mazda), God, was conflated with Good Thinking, Spenta Mainyo, and wrongly counterpoised with Bad Thinking, Ahriman (Angra Mainyo).
In other words, God was set erroneously in opposition to the Devil.
Nevertheless, under the Sassanians, who promoted the clergy’s power, this much more rigid dualism was what the orthodoxy propounded.
To understand the implications of this, we need only think that almost as much time separated the late Sassanians from the original message of Zoroaster, as today the Christian message is separated from its original proponent, Jesus.
In both traditions, the interpretation of the ideas and philosophy have undergone dramatic changes.
Thus just before the Arab conquest in the 7th century CE much knowledge had been distorted, reinvented or lost, so when the fugitives who became known as Parsees left Iran at a time of turmoil, the knowledge which they had at the time and which they sought to preserve was not necessarily "correct" or authentic.
Meanwhile, the Iranians who did not convert to Islam, mainly humble people and priests, struggled in the face of intense persecution to maintain their traditions and the spirit of their religious faith, which did not need texts but had permeated their lifestyles and outlook.