In Persian history we can observe the same regularity: degradation of religious truths some time after it had been received from God.
The essence of the knowledge, which Aryans-Persians possessed since ancient times and which degenerated later on, but were revived by Zarathushtra (Zoroaster), consisted in the following :
There exists Universal God-the-Creator (called Ahura Mazda — in Persian). He manifests Himself as an Infinite Light. He can also be contemplated as the subtlest Goldish Fire.
Forces of good and evil participate in the evolution, but it is Ahura Mazda Who supervises the process.
Man must adhere to three ethic principles: good intentions — good words — good actions.
Cleanness of nature must be maintained even more carefully than that of one’s own body: because everything — all objects and forms — deep inside their multidimensional structure are permeated by the Divine Fire of the Creator. Thus any pollution of nature constitutes desecration of God.
But… as time went by, Persians forgot about the Divine Fire and started to worship ordinary physical fire.
Evil became deified, and people started to believe that a permanent battle was going on in Heaven between the “god of good” and the “god of evil”…
Religious practice got reduced to rituals; for the performance of every single one of them the priests charged a specified amount of money. It was alleged that if a priest was not satisfied with the payment received — the ritual would lose its power… Since numerous rituals were declared absolutely obligatory for each Persian to perform — this resulted in the extraordinary enrichment of the priesthood and the utmost impoverishment of common people.
In addition to this, people were supposed to pay special taxes on the firewood used for large ritual fires (the “holy lights”) that had to be constantly burning. People would buy whole forests of wood in order to maintain them — and all those huge number of trees would get senselessly burnt. 
Ultimately… God brought Muslims to Persia, who destroyed the sanctuaries of Zoroastrians and outlawed all kinds of paganism — i.e. worshipping of fictitious “gods” and idols rather than Universal God.
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But Muslims themselves, both in Persia and in some other countries, did not always remain steadfast devotees of the Creator (Allah in Arabic language). In most cases it was expressed in the fact that aggressive primitive people, who had accumulated coarse personal power and obtained power over people with its help, would start to establish their own tyranny, on the pretext that they attempted to “restore order”.
Formally it looks like an intention to introduce the social traditions that existed in Arabia at the time of the prophet Mohammad. But it has always been just an excuse for establishing a dictatorship of a group of devil-like individuals — with religious free-thinking punished by death and the female part of society suppressed in a cruel manner…
… But such dictatorships make visible those Great Souls that incarnate in order to become — through personal martyrdom — the saviors of whole nations. This happened both in “fundamentalist” Iran  and in several other Muslim countries.
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Let us wish the people of Iran emancipation from the chains of “fundamentalism” and returning to the family of states with sound internal and foreign policies.
Let the number of sound Sufi schools increase in this country, which used to have many good spiritual traditions; let myriads of enlightened sheikhs grow towards God!