Keshab Chandra Sen joined the Brahmo Samaj by 1857 and became an active worker by 1859. He was an impressive speaker and his wonderful ability of explaining the Brahmo philosophy to the common people soon made him immensely popular among the younger members of Brahmo Samaj. He also became a close associate of Debendranath Tagore, who was leading Brahmo Samaj after the death of Raja Ram Mohan Roy. Keshab Chandra Sen and some of his disciples founded the Sangat Sabhas (Believer's Associations) in 1860s. The Sangat Sabhas were small discussion forums that used to meet weekly and discuss about various social and religious issues. However, the disciples of Keshab Chandra Sen in Brahmo Samaj soon started to build a sense of militancy and they abandoned the caste system and also rejected the sacred thread. They rejected to practise temperance as well. Keshab Chandra Sen's followers were desirous of combining belief and action and soon they drove for social radicalism. The number of Keshab Chandra Sen's followers increased significantly during the 1860s.
Keshab Chandra Sen was more liberal in his views compared to the other leaders of Brahmo Samaj and he was an ardent supporter of inter-caste marriage and widow remarriage. He and his followers secretly celebrated an inter-caste marriage in 1862 and they sponsored another inter-caste marriage in 1864. This time they did it publicly and it was also a widow remarriage. This act shocked the more conservative and orthodox Brahmos and a conflict started to begin between Keshab Chandra Sen and the other Brahmos. The conflict between Sen and Debendranath Tagore was broadened in 1865, when Tagore allowed the Brahmos conducting services to wear their sacred threads. Sen objected this decision and withdrew from the Brahmo Samaj along with his followers. Keshab Chandra Sen and his disciples soon established the Brahmo Samaj of India on 15th November, 1866. With this, the first schism of Brahmo Samaj came into existence, as the loyal people to Tagore grouped themselves into the Adi (original) Brahmo Samaj.
Keshab Chandra Sen went out for a long tour to Madras and Bombay and he successfully disseminated the doctrines of Brahmo Samaj there. The members of the English-educated elite in those two major cities were highly influenced with the speeches of Keshab Chandra Sen and soon Brahmo Samajes were established there. Keshab Chandra Sen emerged as a dramatic leader and appealed to the young Bengal Hindus to revolt against the contemporary religion. With his ability to impress people and encourage them, Keshab Chandra Sen soon expanded the network of Brahmo Samaj and total 65 branches of the Samaj were active by 1868 in the eastern section of Bengal. The Brahmo Samaj of India also had 24 missionaries, who were later organised as an 'apostolic body of elders and teachers', namely the Sri Durbar. The missionaries became successful to convince the villagers, who had not been influenced by English education or urban values.
Keshab Chandra Sen went to England during the first half of 1870s and toured, lectured and met Queen Victories during his tour. After returning from England, he put on more emphasis in social action that was intended in restructuring the society and the customs of Bengal. He was instrumental behind the foundation of the Indian Reform Association, which operated with an objective of improving the life of the peasants. With the increasing popularity of Keshab Chandra Sen, his organisation spread in most parts of the subcontinent by 1872 and branches of the Brahmo Samaj were established in the places like Bihar, the United Provinces, Punjab, Assam, Orissa, Madhya Pradesh, etc. The Samaj also spread its influence in the Southern and Western parts of India. There were about 101 Brahmo Samajes existed in India by 1872 and most of them were located outside Bengal. These organisations were being led and ideated by the Bengalis, who had gone there to find employment.
Keshab Chandra Sen strongly demonstrated a new interest in strictly religious issues after 1875 and organised a seminar to study different religions and their prophets. He also argued against the proposal of Durga Mohan that women should be allowed to sit with their relatives during services at the Brahmo temple. Keshab Chandra Sen also began to propagate a new type of Brahmoism during 1875-76 and this Brahmoism included elements of ecstatic religious experience and Shaktism (the worship of female power). He met the famous Hindu religious leader, Ramakrishna Paramahansa during this time and it is thought that Ramkrishna had promoted Sen's involvement in devotional worship. However, as a result of Sen's inclination to devotional worship, many members of Brahmo Samaj disagreed to obey him as leader of the Samaj. This disagreement eventually led to another split in the Brahmo Samaj and the Sadharan Brahmo Samaj was founded on 15th May, 1878.
Keshab Chandra Sen continued his journey towards Bhakti and a universalistic religious ideology and he called upon his supporters to create a new religious revival in February, 1880. He also arranged for a procession through the streets of Calcutta to spread his message more effectively. He tried to synthesise the world's religions and blend elements of different faiths into a single set of rituals and beliefs. Keshab Chandra Sen established the Nava Vidhan (New Dispensation) in January, 1881 and it was symbolised by a red banner that bore the name of his church plus the Hindu trident, the Christian cross and the Islamic crescent. He also introduced the Christian Eucharist, which substituted rice and water for bread and wine.
Keshab Chandra Sen started publishing a newspaper named the New Dispensation in March 1881, to disseminate his ideas and philosophy in a better way. In 1884, the existing newspaper Brahma Dharma was replaced by the Nava Samhita (New Code). Keshab Chandra Sen died at a later half in the year 1884 and his movement also broke down quickly after his death.
Keshab Chandra Sen's ideology had many changes throughout his life and he gave emphasis on different issues in different times. Sometimes, he gave more emphasis on the religious issues and sometimes, he spoke more about the social changes. During the later half of his life, Keshab Chandra took up the role of a religious Guru and preached his own vision of religion. He had a highly charismatic personality and the religious path he took can be defined as new-Vaishnavism. Though, he was responsible for many splits in the Brahmo Samaj including the first schism, he was undoubtedly one of the most discussed and popular social reformers in Bengal during the British period. He also emerged as the key transitional figure in the history of socio-religious movements in India.