In 1638, Roger Williams was expelled from the Baptist church in Boston. The reason for his expulsion is not entirely clear, but it seems to have been due to Williams's insistence on complete separation of church and state. Williams believed that the government should not have any say in religious matters, and vice versa. This was a radical idea at the time, and one that the Baptists were not willing to accept.
Williams left Massachusetts and founded the colony of Rhode Island, where he established a policy of complete religious freedom. This meant that anyone could worship as they please, without interference from the government. This policy made Rhode Island a haven for people who were persecuted for their beliefs elsewhere.