The first occurrence of the name 'Mitra/Mithra' is in a treaty inscription, ca 1400 BCE, established between the Hittites and the Hurrian kingdom of the Mitanni in the area southeast of Lake Van. The treaty is guaranteed by five Vedic gods: Indra, Mitra, Varuna and the twin horsemen, the Ashvins or Nasatya. The Hurrians, it appears, were being led by an aristocratic warrior caste worshipping these gods.
In the Vedic hymns, Mitra is always invoked together with Varuna, so that the two are combined as 'Mitravaruna': Varuna is lord of the cosmic rhythm of the celestial spheres, while Mitra brings forth the light at dawn, which was covered by Varuna. In the later Vedic ritual, a white victim is prescribed for Mitra, a dark one for Varuna. In the Shatapatha Brahmana, the paired One is analyzed as 'the Counsel and the Power,' Mitra the priesthood, Varuna the royal power. In the India Vedic mythology, it is Indra, not Mithra, who slays the chthonic bull. As Joseph Campbell remarked, "Both are said to have a thousand eyes. Both are active foreground aspects of the light or solar force at play in time. Both renew the world by their deed.'
Mitra/Mithra's worship spread across the Iranian plateau and to the Indus Valley civilization and later spread west from Persia to the Roman Empire, where he was called Mithras (q.v.).