Roger Williams was banished from Plymouth after being convicted of sedition and heresy. He had been a Puritan. Puritans were called "Separatists."
Rhode Island was started as a place for religious freedom and for religious dissenters. Roger Williams did not believe that the Plymouth church was sufficiently separated from the Church of England; so you could say that he became is dissenter, but he actually was banished for his beliefs. Anne Hutchinson was also tried, excommunicated and banished from the Massachusetts Bay Colony for her beliefs. She also came to Rhode Island.