Afaik, every pure guidance from God was destroyed by the Persian Sassanid Zoroastrian corrupt priests be it among the Sanatan Dharma, Buddhism, Jewish, Christianity or Islam. The Zoroastrian had this belief that besides themselves none other was a monotheistic believer and they were the only chosen ones by God. Thus the Quran rejects this doctrine and tells actions of all the religious people will be accepted in the sight of God as long as the Good deeds are done, they believe in one god and in the hereafter.
