Born into a Vaiṣṇava vaidya (‘medical’) caste in Calcutta, Sen had a Westernized education. In 1858 he joined the Brāhmo Samāj, its then leader, the brahmin Debendranāth Tagore initiating him as the organization's first non-brahmin teacher (ācārya) in 1862. Subsequently, a group of Brāhmos gathered itself around Sen which was much more radical in its rejection of traditional Hindu practices and attitudes (including adherence to caste distinctions) than Tagore's followers, and took an aggressively missionary stance, drawing on Western models while rejecting the Christianity of the missionaries. The tension between the two groups within the Samāj resulted in the split of 1866, with Sen's followers forming the ‘Brāhmo Samāj of India’. Thereafter, Sen himself became ever more idiosyncratic in his religious practice (a personal synthesis of Christian and Hindu elements), and increasingly indifferent to social reform. In 1870 he visited England, lecturing on the Asiatic nature of Jesus. In 1878 he married his 13-year-old daughter to the 15-year-old crown prince of a British princely state in a traditional Hindu marriage ceremony. This apparent approval of child marriage, and what they regarded as ‘idolatry’, led to Sen's abandonment by many of his followers, who formed the Sādhāraṇ Brāhmo Samāj in 1878. Sen himself gathered the remnant into the ‘Church of the New Dispensation’ in 1881, its practices influenced by Vaiṣṇava devotionalism and Sen's admiration for Rāmakṛṣṇa.