The significant figure in early American history who advocated for religious freedom and tolerance was Roger Williams. As the founder of Rhode Island, he established a colony based on the principles of separation of church and state, allowing for diverse religious practices. Williams emphasized the importance of individual conscience and belief, which contrasted sharply with the prevailing Puritan ideals of his time. His ideas laid the groundwork for greater religious pluralism in America and influenced the development of constitutional principles regarding religious liberties.