Ahura-Mazda (Oromazdos, Ormazd) had created thousands of angelic beings to perform his will and fight on his side against the Evil One; and Alngro-Mainyus (Arimanius, Ahriman) had equally on his part called into being thousands of malignant spirits to be his emissaries in the world, to do his work, and fight his battles.
The greater of the powers called into being by Ahura-Mazda were proper objects of the worship of man, though, of course, his main worship was to be given to Ahura-Mazda.

Angro-Mainyus was not to be worshipped, but to be hated and feared.

With this dualistic belief had been combined, at a time not much later than that of Darius Hystaspis, an entirely separate system, the worship of the elements.

Fire, air, earth, and water were regarded as essentially holy, and to pollute any of them was a crime.

Fire was especially to be held in honor; and it became an essential part of the Persian religion to maintain perpetually upon the fire-altars the sacred flame, supposed to have been originally kindled from heaven, and to see that it never went out.