Brahmo Samaj was established primarily because of the efforts made by Raja Ram Mohan Roy. Ram Mohan was born into a world of diverse cultural influences, as his father was a follower of Chaitanya and his mother worshipped the divine female power. At a very young age, Roy questioned the orthodox beliefs of Hinduism and he published his religious views in a Persian tract, named Tohfat al-Muwabhiddin (A Gift to Deists) in 1804. This was the first time he made public his criticisms of idolatry and polytheism. He strongly opposed the rite of Sati (the immolation of Hindu widows on their husband's funeral pyre). This practice was particularly frequent among the higher castes in Bengal. Before the Brahmo Samaj, Roy attempted to establish an organisational base for his ideas in 1815 by founding the Atmiya Sabha (Friendly Association). This private society can be referred to as the predecessor of the Brahmo Samaj. The members of Atmiya Sabha used to recite Hindu scriptures, sing hymns and hold discussions on religions and social issues, on a regular basis. However, the society was ceased in 1819 and there was no organisation for Roy for the next nine years.
After a long period of nine years, Ram Mohan Roy and his friends (Brajosundar Mitra and others) established the Brahmo Sabha in 1828, on Chitpore Road Kolkata. The Sabha met for the first time on 20th August, 1828. However, it was publicly inaugurated on 11th Magh or 23rd January 1830. The members of Brahmo Samaj celebrate the former date as Bhadrotsab and the latter as Maghotsab. The members of the Sabha used to gather every Saturday evening and first chant the Vedic hymns in Sanskrit. They also used to chant the hymns translated into Bengali, a sermon in Bengali and sing theistic hymns. Though anyone could attend the Sabha, most of the members of Brahmo Sabha were the Bengali Brahmans. There were no norms for membership, creed and no formal organisation for the Sabha during that period.
The main idea of Ram Mohan Roy behind establishing the Brahmo Samaj was to purify Hinduism and to preach the worship of one God. The Samaj was based on those teachings of the Vedas and the Upanishads and also those of other religions, which had some rational basis. The principles of Brahmo Sabha were first sketched by Ram Mohan Roy in a Trust Deed filed in 1830. A reaffirmation of egalitarianism, Roy's concept of the deity, 'the Eternal Unsearchable and Immutable Being who is the Author and Preserver of the Universe', a prohibition of all forms of idolatry and sacrifice and a ban on criticism of other religious beliefs and practices, etc. were included in the Deed. However, Raja Ram Mohan Roy did not elaborate on these principles. He died in 1833.
After the death of Raja Ram Mohan Roy, the organisation of Brahmo Samaj was revived by Maharshi Debendranath Tagore, who also belonged to a Brahman family. He formed the Tattvabodhini Sabha (Truth-Teaching Association), in 1838. It was a society that held weekly religious discussions and monthly worship. Like Roy, he and his society also accepted Vedanta, however, he emphasised more on the superiority of Hinduism. He soon gained a major voice in the propagation of theistic Hinduism. However, though the Tattvabodhini Sabha was quite successful, Tagore decided to revive the Brahmo Sabha.
However, the Brahmo Samaj soon witnessed a lot of conflicts within its members. In 1857, a non-Brahmin social reformer, Keshub Chandra Sen joined the Calcutta Brahmo Samaj and the admittance of Keshub Chandra Sen caused significant stress in the Brahmo movement. Many old Brahmin members started to leave the Samaj and its institutions, mainly due to his dictatorial ways. All these led to the first division of Brahmo Samaj that came into public between August, 1865 and November, 1866. Many small groups started to claim themselves as Brahmo during that period and the most notable among these groups was the "Brahmo Samaj of India". The historians refer this period to as the "First Schism".
The Brahmo Samaj movement is considered one of the most powerful and influential religious movements during the eighteenth century in India. Starting its journey in Calcutta, the Samaj soon spread its idea to the other parts of India. The idea was spread in a period, when the British rulers were expanding the reach of Indian railways all over India and the communication system was also improved a lot. With the help of the easy communication, the Brahmo Samaj movement quickly spread its ideas into the other prominent presidencies like Punjab, Sind, Bombay and Madras. In 1846, a branch of Brahmo Samaj was established in Dacca (present day Dhaka, the capital of Bangladesh) and during the 1850s and 1860s; several young Bengalis got influenced by the Brahmo doctrine.
By the 1860s, the influence of Brahmo Samaj started to spread in South India. A learned and well educated Brahman of Cuddalore, named Sridharalu Naidu became influenced with the ideology of the Brahmo Samaj and he went to Calcutta to know further about the Brahmos. After spending a year studying Brahmo ideology, Naidu returned to South India and dedicated himself to spread the Brahmo philosophy. However, though Naidu did not get significant success initially, a visit of Keshub Chandra Sen to Madras in 1864 helped a lot in the formation of Brahmo Samaj in Madras. The Brahmo Samaj was first established as Veda Samaj in Madras before Naidu transformed it to Brahmo Samaj in both name and content, during the later half of 1860s. Naidu also travelled throughout the South India and helped founding new branches of Brahmo Samaj with able help from Doraiswami Iyengar. The Brahmo Samaj was founded in the places like Salem, Coimbatore, Bangalore, Mangalore, etc.
The Brahmo Samaj has led a number of social and religious reform movements. The Samaj actively participated in almost all social reform movements that include abolition of the caste system and dowry system, emancipation of women and improving the educational system. The Brahmo Samaj also reflected the ideologies of the Bengal Renaissance. The Samaj actively opposed the religious practice of Sati in Hinduism and also supported widow remarriage movement led by Pandit Ishwar Chandra Vidyasagar. In the contemporary period, the Brahmo Samaj is concentrated on making certain social reforms. These reforms include the denunciation of polytheism, reform of educational system, spread of knowledge by universal access to information, legal reform especially in fields of personal and secular law, opposing corrupting influences like intoxicants, television, etc.
The doctrine of Brahmo Samaj was propounded by Raja Ram Mohan Roy and later enriched by the other members of the Samaj. Brahmo Samaj laid emphasis on the worship of one formless omnipresent God. Ram Mohan never claimed that he had founded a new religion different from Hinduism. However, he always opposed the evil practices of Hinduism. Ram Mohan Roy strongly cited scriptural sources to justify his contention that Sati was not required by Hindu law and was instead an erroneous accretion. He described Sati as an example of degenerated Hinduism. Roy was adhered to theism and to him; God and his presence were proven by the complexity of reality. Roy envisioned God as the "almighty superintendent of the universe". He also strongly opposed the debarring of women from education and believed that the elaborate and useless rituals, idolatry and polytheism should disappear. There are certain doctrines of Brahmo Samaj that are still followed by the Brahmos. The members of Brahmo Samaj have no faith in any scripture as an authority; they have no faith in Avatars; they denounce the polytheism and idol-worship; they oppose the caste restrictions; and they also make faith in the doctrines of Karma and Rebirth optional.
The Brahmo Samaj has played a significant role in the renaissance of India and the roots of much of the modern thinking in India can be traced back to the Brahmo Samaj movement. Most of the renowned and eminent social reformers and thinkers of the eighteenth century were directly or indirectly connected to the Brahmo Samaj. The Nobel Laureate, Rabindranath Tagore was one of the luminaries of the Brahmo Samaj.