At the time of Alexander the Great, Greeks conquered a large part of Asia, from Asia Minor to modern- day Afghanistan, Pakistan and the north east of India. Greek influence continued after Alexander´s death as a number of Greek Kingdoms were founded in Asia.
During this time of Greek influence in the region, a mix between Greek and Buddhist faith was adopted by Greeks and locals. Alexander the Great and his descendants never tried to impose the Greek Faith or culture by violence. They intervened only in cases when they would consider the local customs as brutal or uncivilised. Buddhism coexisted in this context as well, and its followers never tried to dominate over the local culture of each area. Buddhism pointed out any negative local practices and encouraged the positive ones.
When Alexander the Great arrived in central Asia, Buddhist monks were already active in the region. At that time, Greeks were referred to as the Yonas (originating from the Greek word for Ionians, one of the four major Greek tribes; others being the Aeolians, Achaians and Dorians). Buddhism was well-received and spread quite rapidly among the Greeks of Alexandria on the Caucasus and Taxila, to the point that in Asoka’s time these cities were considered as main Buddhist centers. Greco-Buddhism did not take over 100%, as the traditional Hellenic religion still had a significant number of followers.
In the Mahavamsa, a chronicle dedicated to the kings of Sri Lanka, the high monk present in the devotion of the high Stupa ( a hemispherical structure containing relics and used as a place of meditation) was a Yona (Greek) monk called Mahadhammarakkhita who brought 30,000 Greek monks from Alexandria of the Caucasus to join him. A monk with the same name was sent north by Asoka to spread the rock edicts in the Greco-Bactrian lands. Also, according to the Mahavasma, early Buddhists in Sri Lanka went to Alexandria of the Caucasus to learn more about Buddhism.
Evidence of Buddhist expansion during the Indo-Greek period (starting in 180 BCE) comes from coins;also, we see a significant number of Stupas built during that period and statues depicting the Buddha, often accompanied by Greek Gods and Goddesses. In Greco-Buddhist culture and art, Greek Gods are considered as guardians to the Buddha or are represented as Devas accompanying Buddha during important events of his life.
The religion which was practiced by the Indo-Greeks who followed Buddhist philosophy but who continued to adhere to the Greek customs and cultures is known as Greco-Buddhism. Greco-Buddhism is a form of Hellenism which adapts Buddhism into itself, or a form of Buddhism that embraces the Hellenic culture. Greco-Buddhism can be seen in two ways. Firstly, as the cultural and religious syncretism between the Hellenic culture and Buddhism. On the other hand, it also corresponds to the adoption of the Buddhist philosophy by a person with a Hellenic background.
The ancient Greek belief system did not prevent people from learning a new philosophy or worshiping a God unknown to the Greek pantheon, if they were to go to a different country. However, the Greeks would adhere to a set of rules and customs. As Buddhism became more and more integrated into the life of the Indo-Greeks, some Greek Gods like Heracles and Tyche were seen as guardians of the Buddha and thus, guardian Gods of Buddhism.
Greco-Buddhism gave a human form to Buddha through statues based on a Greek style. Before any such statues were made, the Buddha was symbolised by the Dharma Wheel or a Stupa. Also, Greco-Buddhism is said to have paved the way for the propagation of Mahayana Buddhisms (one of the two main branches of Buddhism; the other being Theravada) to China and Japan.
A number of Greek Gods formed part of Greco-Buddhism. Most of these deities were considered as protectors of Buddha. Such deities include Hercules (ΗΡΑΚΛΗΣ; syncretized with Vajrapani), Tyche ( ΤΥΧΗ, syncretized with Hariti), Zeus (ΖΕΥΣ; associated with Indra), Boreas (ΒΟΡΕΑΣ; the wind god) and Athena ( ΑΘΗΝΑ).
However,the Buddhist Greeks were still Greeks and definitely wanted to honour and venerate the Gods of Greece. The Greeks presumably considered that from a Buddhist/Greek context that honouring the Gods is a form of sowing good and that offering to the Gods secured a reciprocal relationship with the Gods.