Overview of the Persian Empire
The Iranians were originally polytheistic. They had a number of gods, the most important being Ahuramazda who was the benefactor of all living beings. They also however believed in the sun god as well as a host of other dieties. Fire became a source of prayer.
Many Persians took up the beliefs of Zoroaster. Zorasterism believed there was a constant battle between good and evil. Good was embodied by the god Ahuramazda and evil by the god Ahrimamn.. According to Zoraster man has the choice of choosing either, and will be judged on judgement day on whether he or she will go to heaven or hell, depending on what path they have picked. Zorosterism gain the support of Emperour Darius, and although he did not try to impose it on the inhabitants of the Empire his support insured its wide spread.