Why was Mithra considered a guardian of justice and truth?

In ancient Iranian religion, Mithra was considered a guardian of justice and truth primarily because he was the god of the covenant. The word mithra itself functioned as a common noun meaning "contract," "treaty," or "covenant," establishing his role as the celestial deity who oversaw all solemn agreements made between individuals, countries, or other sociopolitical entities.1

Mithra’s authority in the ethical sphere was absolute. He was described as an "ever-waking" protector with 10,000 eyes and 1,000 ears, allowing him to vigilantly monitor human interactions and identify those who broke their word. He acted as a stern judge who severely punished anyone who violated the terms of a covenant, often intervening in conflicts to ensure the defeat of treaty breakers, whom he cast into panic. By protecting the territories of those who remained faithful to their agreements, Mithra served as a divine guarantor of social order, integrity, and truth.1