Owing to its ethnic and cultural diversity, religion in Arunachal Pradesh has been a spot for the syncretism of different traditional religions. Much of the native populations follow indigenous religions which have been systematised (generally following Hindu models) under the definition "Donyi-Polo" (Sun-Moon) since the spread of Christianity in the region by Christian missionaries in the second half of the 20th century. The province is also home to a substantial Tibetan population in the north and northwest who follow Tibetan Buddhism, of ethnic groups who subscribe to Hinduism, and other religious populations. Christianity is followed by over 30% of the population, mostly by natives.
Christianity now represents about one-third of the total population. Christians, predominantly belonging to the Catholic Church, were 0.71% of the population in 1971, 10.30% in 1991 and 30.26% in 2011. According to the Swarajya magazine, the conversion of the population has been carried out through "allurements, blandishments, blatant bribery and even by force" and supported by the rule of Rajiv and Sonia Gandhi through the 1980s, 1990s and 2000s.
Prior to the 1980s, Jawaharlal Nehru and his daughter Indira Gandhi affirmed that it was strategically important that Arunachal Pradesh populations were not coaxed into Christianity, so that in 1978 a law against forced conversion through physical and psychical threat was passed. However, it was never put into practice, despite the tools for conversion used by Christian missionaries were well known, and later Rajiv Gandhi's government in New Delhi pressured Gegong Apang (the then Chief Minister of Arunachal Pradesh) to allow Christian missionaries to ingang the state. Donyi-Poloism emerged as a self-conscious organised movement during the same years, with the aim to systematise the indigenous religions in order to prevent the takeover of Christianity.
Through the following decades a "range of inducements, including admissions in schools run by the missionaries, admissions and free scholarships in colleges run by them in other parts of the country, free healthcare, regular doles in the form of financial assistance and material gifts as well as blatant threats were employed by the proselytisers". In 2011 the new Chief Minister of Arunchal Pradesh, the Catholic Nabam Tuki, was handpicked by Sonia Gandhi herself. Tuki deployed the state bureaucracy to help the Christians, making them the sole beneficiaries of social welfare schemes. Tuki also prevented Hindu organisations like the Ramakrishna Mission and the Vanvasi Kalyan Ashram, who work for the preservation of indigenous faiths, to work among the tribals. Tuki slaughtered a bovine at the behest of Christian priests, and Sonia Gandhi pressured to keep him in power even after the 2016 political crisis in Arunachal Pradesh, when he lost the support of the legislative assembly and his links with the National Socialist Council of Nagaland were exposed.
According to Nani Bath, a professor of political science in the Rajiv Gandhi Central University at Naharlagun near the state's capital Itanagar, "Christian missionaries find it easy to lure away our people with allurements and material as well as financial enticements. The Christian missionaries also offer false hopes of salvation". According to a state intelligence officer, since the fall of Tuki, Christian churches have become more politicised, delivering sermons not only against the indigenous religion Donyi-Polo but also against Hinduism and the Bharatiya Janata Party, and trying to convince Arunachal tribals to join Nagaland in order to form a Christian state.
Migration too has been playing a major role in the changing of the religious demography of Arunachal Pradesh. The coming of Muslim populations in search of jobs and better livelihood has contributed to the growth of an Islamic community.