Mitra, a Proto-Indo-Iranian language language, whose nominative form is Mitras, is an ancient Indo Iranian deity. This god was originally a common object of worship in the Aryan pantheon. After the differentiation of Iran Aryan and Indo-Aryan peoples, it began to develop towards different characteristics (Mitra in Avesta and Mithara in Veda).

In some ancient languages of the Indo-Iranian languages (such as Sanskrit and Avestan), "mitra -" can be interpreted as "contract" or "companion" (the root "mi -" means "constraint", and the suffix "- tra" means "tool"). However, Sanskrit emphasizes the concept of "companion" more than Avestan, so the word "mitra" can also be used as a common noun "friend" in Rigveda, in addition to being used as a god name and a proxy for Mithara. The situation in Iranian language is the opposite, with a greater emphasis on the concept of "contract".
Modern researchers generally believe that the usage of the Iranian language is closer to the original appearance of the word. French scholar A. Mayer first proposed that the original meaning of Mitra's god name was contract, so Mitra's original function should be the god of contract. His viewpoint has been recognized by most scholars. In fact, although Mitra later developed into the sun god, the god of light and even the god of war, his nature as the god of contract has not disappeared. There is evidence that many worshippers still regard Mitra as the guardian of contract after the A.D.

Early worship
Mitra is an extremely ancient god belonging to the Aryan religious system (some scholars call it the primitive Indian Religion in Iran religious system). It is impossible to investigate the worship of Mithras in prehistoric times before the Aryan division. The oldest existing record of Mitra is a 14th century BC Hittite language inscription, which is a peace treaty signed between Hittite and Mitanni, located in the southeast of Van Lake in modern Türkiye. The inscription refers to Mitra (in the form of mi it ra) as the main god of Mitanni.
Mitanni is a country with a special background. Most of its residents are Hulites, while the ruling class is Aryan. It is generally believed that Mitanni's ruling group is homologous with Indo-Aryan peoples people. It is mentioned in the inscription that Mitra is one of the five great gods worshipped by Mitanni people, while the other four gods obviously correspond to Varuna, Indra and Ashvins (Ashvins are twin gods) in Vedas.
Early Indo Iranian documents still retain the original style of Mitra God, and the descriptions of Mitra Mithra in Rigveda and Avesta are very similar. In fact, scholar William Jackson gave an example where the records of Rigveda and Avista are almost identical:
A Mitra ode from the Yasht part of Avesta:

tam amavan tam yajatam

suram dhamasu savistham

mitram yajai ho trabhyah

(The main idea is: This is the powerful Mitra God, the strongest of all creatures. I would like to offer wine.)
The literature shows that the ancient Indians usually worship Mitra together with another great god, Varuna. These two gods belong to the ancient gods of Adido (meaning "God without hindrance", which is the general name of the descendants of Aditi and Kashyapa), and there are some overlaps in their duties. Vedas often refer to "Varuna Mithara", sometimes they simply regard them as a god, and seldom mention Mithara alone. In the whole Rigveda, there is only one poem extolling Mithara alone, and other hymns dedicated to Mithara are included in the hymns extolling Varuna. The situation of this dual god (Varuna Mitoro) also exists in the ancient Iranian myth system. In Avesta, the god corresponding to Varuna and forming a dual god with Mitra is only Apam Napat (the god of water).

It is likely that before Zoroaster's Reformation, Mitra worship was widely popular among Iranians. It is known that Mitra is one of the only three great gods in Avesta that can be called Ahura (the other two are Ahura Mazda and Apam Napat). Zoroaster's religious philosophy was based on making Ahura Mazda the highest god, so Mitra's status was greatly degraded. Mitra was never mentioned in the oldest part of Avesta, "Gata". However, the later Zoroastrian priests restored their respect for Mitra for some reason. The later part of the Avesta (as opposed to the Gata which is believed to be written by Zoroaster himself), "Yasht", contains a large number of hymns about Mitra.

Zoroaster
Zoroaster (Zoroastrianism) is the product of Zoroaster's reform of primitive Religion in Iran. The obvious result of the reform is the introduction of religious Dualisms, which shows the world as a struggle between the good god (Ahura Mazda) and the evil god (Angela Manuel); It also changed the original multi god system, making Ahura Mazda the only god, and other gods handed down from ancient times became angels. This has greatly impacted Mitra's position. The name of Mitra is not mentioned in the gata written by Zoroaster himself.
In the later Jeshite written by later generations of priests, Mitra was shown to belong to the creature like other angels ("I created him", Ahura Mazda said to Zoroaster, "and made him deserve sacrifice and prayer like myself"). Mitra is called "the judge of the soul" in Yeshite, and looks after human affairs together with his creator Ahura Mazda. Even so, Mitra is not an important god. He is not even among the top six angels. Mitra occupies only a middle position in the hierarchy of Zoroaster's gods, and is one of the greatest Yazata (the so-called Yazata is an assistant created by Ahura Mazda to help fight against evil gods and manage the world).

Mitra is the representative of Ahura Mazda in the world, defending truth and justice from the evil forces of Angela Manuel. But Mitra has always been an abstract god, only sometimes associated with light. Similar to the original function of defending contracts and punishing those who violate contracts in the Prehistory, Mitra in Zoroaster is a symbol of truth and loyalty, rewarding good things and attacking bad things.

Mithraism
After Mitra of Culture of Iran was accepted by Greek Roman culture, a new belief was formed: Mithraism. Mithraism is a Mysticism religion, a strange mixture of eastern and western cultures. Mitra first entered the vision of the Greeks in the Hellenistic period after Alexander the Great's Eastern Expedition. The Greeks equated him with Helios, the sun god in Greek mythology (it is the tradition of the Greeks to associate the gods of other religious systems with their own gods). About the second century ago, Mitra Helios became the main god of Mithraism, which was probably completed in Pergamon, Asia Minor. The Romans were already aware of the existence of this sect at least when Cornelius Pompeii fought in the East. This novel belief was introduced to Rome in the 1st century BC, and expanded rapidly after Rome entered the imperial era. Emperor Commodus and the soldier emperors in the crisis of the third century both favored Mithraism, because it had the nature of a "warrior class religion". The fact is that Mitra worship was widely popular in the Roman army and became the common belief of soldiers, and extended to the marginal areas of Britain and the Rhine.

It is difficult to reconstruct the form of Mithraism, because its worship ceremony is highly mysterious (comparable to the so-called Mysteries in ancient Greece) and the believers' groups are highly exclusive (only recommended men are accepted). To be sure, the nature of its Mitra is different from that of the original Indo Religion in Iran religion and the Zoroastrian Mitra. In Mithraism, the image of Mitra is "cattle slayer", which does not appear in any Culture of Iran system. Perhaps Mitra here is equivalent to the protector of God of War or soldiers. Some scholars believe that Mithraism was strongly influenced by Babylonian Astrology.
The worship ceremony of Mithraism is held in natural or artificially excavated caves, called shrines, which are said to imitate Mitra's birth environment. There are many temples located underground of existing large buildings. There are also many ruins of such temples in Italy and Germany.

The relationship between Mithraism and Christianity is a topic of interest to many scholars. At the height of Mitra's belief in the 3rd century AD, the two religions competed widely for believers. The relationship between Christmas and Mitra is the most peculiar, because this festival (December 25 according to the current Gregorian calendar) was originally the birthday of Mitra in the Roman Empire. The New Testament and some of the oldest Gnostic texts do not record the birth date of Jesus, and the date of Christmas was actually established by the church in the 2nd century. Saint Augustine of Hippo pointed out the unpleasant relationship between Christmas and pagan festivals. Many modern and contemporary researchers (such as James George Frazer, the author of Jinzhi) believe that the Early Christianity Church deliberately chose this date in order to better attract pagans to Christianity. In short, in the struggle between Christianity and Mithraism, Mithraism failed in the end, but it cannot be considered that it did not leave any influence.

The worship of Mitra God is widespread in areas affected by Culture of Iran. The Anshi people (Parthia people) belong to the Eastern Iranian languages, and they nomadized in Central Asia before entering Iran itself. They are strongly influenced by Culture of Greece, but they still belong to the Iranian system in religion. Many of the resting kings were named "Mithradates" (meaning the son of Mitra), which shows that Mitra faith plays an important role in the spiritual life of the rest people.
In Asia Minor and Armenia, historians know that the princes here once believed in Mitra, but whether this belief has a strong influence on the people cannot be determined. The Kingdom of Pontus is a typical example. It is an area influenced by both Culture of Greece and Culture of Iran. Many princes have the name "Mithradates", including the famous Mithradates VI. More distant regions, including Central Asia and North India, also had Mitra faith due to the expansion of Culture of Iran. In the coins issued by the Kushan Empire, archaeologists found the name and image of Mitra.