The Brahmo Samaj's central theological claim, that the Upanishads teach an unqualified monotheism compatible with reason, was itself a matter of debate from the movement's earliest years. Critics questioned how much of Roy's reading reflected the Upanishads on their own terms and how much reflected his engagement with Christian Unitarian thought and Islamic monotheism, both of which he had studied directly and admired for their rejection of image worship. Roy himself argued that his conclusions were an authentic recovery of Vedantic teaching rather than an import, a position his successors continued to defend.
The movement's internal history is largely a history of disagreement over how far reform should go. Debendranath Tagore's Adi Brahmo Samaj retained more continuity with Hindu ritual practice and caste-conscious membership, while Keshub Chandra Sen's Brahmo Samaj of India pushed toward a more universalist religion, open intercaste and interfaith marriage, and eventually toward practices some members regarded as no longer recognizably Hindu at all, which precipitated further splits after Sen's death.
However members later evaluated its specific theological claims, the Samaj's practical legacy is not seriously disputed: it trained a generation of reformers, several of whom went on to found or lead other movements, and it placed questions of caste, idol worship and women's status at the center of Indian public religious debate in a way earlier reform efforts had not managed on a comparable scale.