Complex sentence: Thakur, whose family were Vaishnavite Hindus, founded a sect of Vaishnavite Hinduism called Matua Mahasangha, which was adopted by members of the Namasudra community who were also known by the pejorative name of Chandalas.
Split into simpler sentences: Thakur, whose family were Vaishnavite Hindus, founded a sect of Vaishnavite Hinduism called Matua Mahasangha. This was adopted by members of the Namasudra community, who were then also known by the pejorative name of Chandalas.