The Lepchas are considered to be the earliest inhabitants of Sikkim. However the Limbus and the Magars also lived in the inaccessible parts of West and South districts as early as the Lepchas perhaps lived in the East and North districts. The Buddhist saint Padmasambhava also known as Guru Rinpoche, is said to have passed through the land in the 8th century.The Guru is reported to have blessed the land, introduced Buddhism and foretold the era of monarchy that would arrive in Sikkim centuries later.

Every aspect of the society in Sikkim has a strong bearing upon religion.

Hinduism is the state's major religion and is practised mainly by ethnic Nepalis; an estimated 60% of the total population are adherents of the religion. There exist many Hindu temples. Kirateshwar Mahadev Temple is very popular, since it consists of the chardham altogether.

Vajrayana Buddhism, which accounts for 27% of the population, is Sikkim's second-largest, yet most prominent religion. Prior to Sikkim's becoming a part of the Indian Union, Vajrayana Buddhism was the state religion under the Chogyal. Sikkim has 75 Buddhist monasteries, the oldest dating back to the 1700s. The public and visual aesthetics of Sikkim are executed in shades of Vajrayana Buddhism and Buddhism plays a significant role in public life, even among Sikkim's majority Nepali Hindu population.

Christians in Sikkim are mostly descendants of Lepcha people who were converted by British missionaries in the late 19th century, and constitute around 10% of the population. As of 2014, the Evangelical Presbyterian Church of Sikkim is the largest Christian denomination in Sikkim. Other religious minorities include Muslims of Bihari ethnicity and Jains, who each account for roughly one per cent of the population. The traditional religions of the native Sikkimese account for much of the remainder of the population.

Although tensions between the Lepchas and the Nepalese escalated during the merger of Sikkim with India in the 1970s, there has never been any major degree of communal religious violence, unlike in other Indian states. The traditional religion of the Lepcha people is Mun, an animist practice which coexists with Buddhism and Christianity.

In North-Sikkim, Tibetan buddhism has its strongest implantation (over 90%); ruled by the Lamas who are teaching the Dharma, which is the doctrine pertaining to the purification and moral transformation of human beings.

Catholics are present in South-Sikkim but not in northern district. Sikkim is, as Bhutan, part of the Diocese of Darjeeling, which pastorally served, in 2014, 37,109 Catholics (2.6% of 1,433,000 total) on 9,521 km² in 54 parishes and 3 missions with 132 priests (82 diocesan, 50 religious), 455 lay religious (121 brothers, 334 sisters) and 40 seminarians.