Rhode Island's founding is unusual among the thirteen colonies for having a specific argument behind it rather than a land grant or a trading charter. Roger Williams was banished from Massachusetts Bay in 1635 for preaching that civil magistrates had no authority over matters of conscience, what he called soul liberty, and rather than recant he walked south and in 1636 bought land from the Narragansett to found Providence on exactly that principle. The colony he built was not tolerant in the qualified, one denomination at a time sense several other colonies eventually became. It was constitutionally opposed to any religious establishment at all from its first charter, a position roughly a century and a half ahead of the First Amendment that would eventually generalize it to the whole country.
The institutions that followed prove the principle was load bearing rather than rhetorical. The First Baptist Church in America, which Williams helped found in Providence in 1638, gave Baptists a colony to organize in when they were harassed elsewhere in New England. The Touro Synagogue in Newport, dedicated in 1763, is the oldest surviving synagogue building in the country, built by a Sephardic Jewish community that had settled in Newport specifically because Rhode Island would let them. Historians of the establishment clause treat Rhode Island's charter as one of the direct intellectual ancestors of the First Amendment's religion clauses, not a colorful precedent but an argument the framers had already seen work for a century and a half.