In the second millennium BCE, on the vast Iranian plateau, an ancient religion emerged that later became known by its founder, Zarathustra Spitama. Zoroaster, the ancient prophet of the Iranians, propagated his religion in the eastern region of the Iranian plateau, especially in the areas of Khwarezm (Khorasan). This religion, later known as Zoroastrianism worldwide, is one of the world's first religions that showed a clear inclination towards monotheism and incorporated profound philosophical thoughts.
At the core of this religion was the worship of Ahura Mazda ; a transcendent being called "Wise Lord". Ahura Mazda was the sole creator of the universe and the source of all goodness, light, and brightness. He was the originator of cosmic order and the law of truth, which in Avestan is called "Asha". Ahura Mazda, timeless and spaceless, was the source of wisdom and benevolence, overseeing all His creations and bestowing good and blessings upon the world.
Zoroastrian ontology was based on a fundamental dualism, but not a dualism of two gods. This dualism was between two spirits or two intellects that originated from Ahura Mazda: Spenta Mainyu (the Bounteous Spirit or Holy Intellect) and Angra Mainyu (the Opposing Spirit or Evil Intellect). Spenta Mainyu symbolized goodness, creativity, truth, and light, while Angra Mainyu symbolized evil, destructiveness, falsehood (druj), and darkness. The battle between these two spirits was a cosmic struggle that unfolded in all dimensions of existence.
In this cosmic battle, humans possessed complete freedom of will and choice. Zoroaster taught his followers that each individual must consciously choose the path of truth (Asha) and stand against evil forces with good thoughts, good words, and good deeds (Humata, Hukhta, Huvarshta). This emphasis on individual responsibility and moral choice was a unique characteristic of Zoroastrianism in ancient times and granted humans an active role in the destiny of the world.
In Zoroastrianism, other important concepts also existed. The Amesha Spentas (Holy Immortals) were six attributes or manifestations of Ahura Mazda who assisted Him in governing the world, and each symbolized an aspect of God's being. Also, the Yazatas (Worthy of Worship) were deities who served Ahura Mazda, and each was a guardian of a natural element or a moral concept. Mithra (Guardian of Covenant) and Anahita (Guardian of Waters) were among the most important Yazatas.
Fire, in Zoroastrianism, was revered as one of the most important symbols. Fire was a symbol of Ahura Mazda's light, purity, brightness, and truth, and it was always kept lit in Zoroastrian temples. Zoroastrian rituals and prayers were often performed in the presence of fire and with adherence to the principles of physical and spiritual purity. This religion considered life a divine gift and taught humans to protect the elements of nature and strive to increase goodness in the world.
The influence of Zoroastrianism on Iranian kingdoms, especially the Achaemenids, was very profound. This religion emphasized justice, truth, order, and governance based on the will of Ahura Mazda. Achaemenid kings considered themselves representatives of Ahura Mazda on earth and believed their duty was to spread peace and justice in the world. These teachings became the moral foundation of the Achaemenid government and were reflected in their policies of tolerance.