Bangladesh is a secular and Democratic country as per constitution.  Although removed from the constitution once, it was later reinstated. Islam is the largest religion of Bangladesh; Muslims constitute over 90% of the population, while Hindus constitute 8.5% and Buddhists 0.6% are the most significant minorities of the country. Christians, Sikhs, animists and atheists form 1% of the minuscule remainders.[self-published source] A survey in late 2003 confirmed that religion is the first choice by a citizen for self-identification. Bangladesh only recognises Islam, Christianity, Hinduism, and Buddhism.
The Muslim population in Bangladesh is 146 million which makes up 90% percent of the population in the country. Bangladesh is the fourth largest Muslim populated country.  Muslims are the predominant community of the country and they form the majority of the population in all eight divisions of Bangladesh. Overwhelming majority of Muslims in Bangladesh are Bengali Muslims at 88%, but a small segment about 2% of them are Bihari Muslims and Assamese Muslims. Most Muslims in Bangladesh are Sunnis, but there is a small Shia community and an even smaller Ahmadiyya community. Most of those who are Shia reside in urban areas. Although these Shias are few in number, Shia observance commemorating the martyrdom of Muhammad's grandson, Husain ibn Ali, is widely observed by the nation's Sunnis. Muslims celebrate Eid ul-Fitr, Eid ul-Adha, Muharram, Milad un Nabi, Shab-e-Barat and Chand Raat all across the country with much fanfare and grandeur. The annual Bishwa Ijtema is the largest and most notable congregation of Muslims in Bangladesh.
The Muslim community in the Bengal region developed independent of the dominant Islamic trends in India. Features of Bangladeshi Hinduism, which differed in some respects from Hinduism in other parts of South Asia, influenced both the practices and the social structure of the Bangladeshi Muslim community. In spite of the general personal commitment to Islam by the Muslims of Bangladesh, observance of Islamic rituals and tenets varies according to social position, locale, and personal considerations. In rural regions, some beliefs and practices tend to incorporate elements that differ from and often conflict with orthodox Islam.
Hinduism is the second largest religious affiliation in Bangladesh, with around 14 million people identifying themselves as Hindus. Hindus make up about 8.5% of the total population. In terms of population, Bangladesh has the third largest Hindu community of the world, after India and Nepal. Bangladeshi Hindus are predominantly Bengali Hindus, but a distinct Hindu population also exists among the indigenous tribes like Garo, Khasi, Jaintia, Santhal, Bishnupriya Manipuri, Tripuri, Munda, Oraon, Dhanuk etc. Hindus are evenly distributed throughout all regions of Bangladesh, with significant concentrations in northern, southwestern and northeastern parts of the country. In nature, Bangladeshi Hinduism closely resembles the rituals and customs of Hinduism practised in the neighbouring Indian state of West Bengal, with which Bangladesh (at one time known as East Bengal) was united until the partition of India in 1947. Hindu festivals of Durga Puja, Rath Yatra and Janmashtami witness jubilant celebrations across various cities, towns and villages of Bangladesh.
In antiquity, the region of present-day Bangladesh was a center of Buddhism in Asia. Buddhist civilisation, including philosophies and architecture, traveled to Tibet, Southeast Asia and Indonesia from Bengal. The Buddhist architecture of Cambodia, Indonesia and Thailand, including the Angkor Wat Temple and the Borobudur vihara, are believed to have been inspired by the ancient monasteries of Bangladesh such as the Somapura Mahavihara. Strange though it may now seem in such an overwhelmingly Muslim country, Buddhism has been no small player in the nation's history and culture.
Most of the followers of Buddhism in Bangladesh live in the Chittagong division. Here, Buddhism is practised by the Bengali-speaking Baruas, who are almost exclusively Buddhist and are concentrated heavily in the Chittagong area as well as few of the Barua Buddhists live in other parts of Bangladesh, such as Comilla, Mymensingh, Rangpur, Sylhet districts. Most of the followers of Buddhism in Bangladesh live in southeastern region, especially in the Chittagong Hill Tracts, Chittagong and Comilla district. Most of the Buddhists of Chittagong Hill Tracts belong to the Chakma, Marma, Mru, Khumi, Bawm, Chak, Kuki, Murang, Tanchangya and Khiang tribes, who since time immemorial have practised Buddhism. Other tribal communities who practise animism, have come under some Buddhist influence. The beliefs and rituals of the Buddhist communities in this region are amalgamations of Buddhism and ancient animistic faiths. Buddha Purnima is the most widely observed festival among both Bengali Buddhists and Buddhist tribes.
Christianity arrived in what is now Bangladesh during the late sixteenth to early seventeenth centuries AD, through the Portuguese traders and missionaries. Christians account for approximately 0.4% of the total population and they are mostly urban community. Roman Catholicism is predominant among the Bengali Christians, while the remaining few are mostly Protestants. Few followers of Christianity are also present among certain indigenous tribal communities such as Lushei, Bawm etc.
There are approximately 100,000 people adhering to the religion of Sikhism. The presence of this religion goes back to the visiting of Guru Nanak at 1506-07 with some of his followers to spread Sikhism in the region of the present day Bangladesh. When some Bengali people accepted this faith then a Sikh community was born. This community had become bigger when almost 10,000 Sikhs came from India during the Bangladesh Liberation War. This community had made a great progress to the country. Today there are almost 10 gurdwaras in Bangladesh. Among them only 7 are well-known especially the Gurdwara Nanak Shahi beside the University of Dhaka in Dhaka which was built in 1830, the most oldest gurdwara in Bangladesh.
Although Bangladesh initially opted for a secular nationalist ideology as embodied in its Constitution, the principle of secularism was subsequently replaced by a commitment to the Islamic way of life through a series of constitutional amendments and government proclamations between 1977 and 1988. The Constitution establishes Islam as the state religion. But also there is secularism in Bangladesh which recently came back. The Government generally respects this provision in practice; however, some members of the Hindu, Christian, Buddhist, and Ahmadiyya communities experience discrimination. The Government (2001-2006), led by an alliance of four parties (Bangladesh Nationalist Party, Bangladesh Jamaat-e-Islami, Islami Oikya Jote and Bangladesh Jatiyo Party) banned the Ahmadiyya literature by an executive order.
Family laws concerning marriage, divorce, and adoption differ depending on the religion of the person involved. There are no legal restrictions on marriage between members of different faiths.
In 2010, secularism was restored, but Islam remained the state religion per Article 12.
There have been several instances of violence against the religious minorities in Bangladesh. Hindus, Buddhists and Christians have come under widespread attacks by Islamist extremists during communal riots, elections and post-poll violence. However, most of these violences are perpetrated primarily against Hindus, the largest minority of the country, who are particularly vulnerable in a period of rising violence and extremism, whether motivated by religious, political or criminal factors, or some combination. Bangladesh has been rocked by several anti-Hindu riots in 1992, 2001, 2013 and 2014. These violences included attacking and killing Hindus, looting and burning of Hindu-owned properties and businesses, abduction and rape of women, desecrating and destroying Hindu temples by the extremist Muslim mobs. There are also alleged discrimination against Hindus by the administration in the form of Vested Property Act by which over 40% of Hindu-owned lands and houses have been confiscated, intimidation during elections and revoking their names from electoral rolls. Since the rising of Islamist political parties during 1990s, large number of Hindu families have migrated from Bangladesh to India due to a sense of insecurity and economic necessity. These factors combined with lower birth rates of minorities have resulted in a dwindling Hindu population in the country. The Bihari ethnic minority in Bangladesh has been subject to persecution during and after 1971 Liberation War. Due to their pro-Pakistan stance, many Biharis were forcefully repatriated to Pakistan and those who stayed back were not granted citizenship and voting rights by Bangladesh government.
There have been multiple attacks and murders of atheist bloggers, academics and authors by Islamist militants since 2013, with the government accused of being unable or unwilling to provide protection - and in some cases even persecuting atheists and imprisoning them.