Just how Puritan was early colonial New England? The first legal code, The Body of Liberties, was written by a minister.

Among the offenses that could get you put to death: worshipping another god, blaspheming, and committing adultery. Sure, the separation of church and state, and your right to religious freedom, are bedrocks of the American way of life  today— but it hasn't always been like that.

"The Massachusetts government was almost a theocracy," said John Barry, author of "Roger Williams and the Birth of the America Soul." "Everybody in it believed that the goal of Massachusetts’s government was to fulfill God’s will."

It was this theocratic world that Roger Williams stepped into, when he arrived in Boston from England in the winter of 1631.

"He was a Puritan," Barry said. "In fact, the day he arrived from England he was offered a job as minister at the Puritan church in Boston, which he declined because he didn’t think they were pure enough for him."

Barry says that it was largely Williams’ desire to protect the purity of the church that drove him to an inexorable conclusion: A firewall needed to be built between the church and the state.

"He viewed the church as something so pure an apart from the world, he felt that any connection between the church and the world would have to contaminate the church," Barry said.

And Williams didn’t stop there. He was well schooled in the law, worldly, and intellectually sophisticated. He carried the idea of this separation to its logical end. If the goal of the state was not to implement God’s law, then the state must also tolerate other gods — or, indeed, no god at all.

"He recognized that plenty of other governments that functioned under other religions were thriving economically, and that the idea that God blessed one and punished another didn’t make any sense," Barry said.

And so he challenged the validity of the colonists' charters in America, which were based in part on the notion that the king of England was the first Christian monarch to have discovered the lands. And despite the fact that, as a man of his times, he saw Native Americans as inferior, Williams sought to understand them, learned their language, and bristled at the colonists shaky legal justifications for procuring their land.

"All Christians believe that all people are equal," Barry said. "But he didn’t simply pay lip service to the idea he actually did believe it."

None of this sat well with authorities in New England, made all the worse by the fact that Williams was doing it in defense of his Puritan faith. This wasn’t some outsider undermining the very foundations of their burgeoning new world. He was one of them.

"The authorities in Massachusetts recognized that his reputation was such, that’s what made him so dangerous," Barry said. "That people respected him and knew he was devout."

So on Oct. 9, 1635, the General Court of Massachusetts took action, banishing him from the colony for sedition and heresy. He traveled south of the Seekonk River, leaning on his relationships with the natives, and his connections in England, to secure a charter for a new colony. He called it Providence.

"In the founding document for Providence, he did not even ask for God’s blessing on the endeavor because he felt it was an entirely human endeavor," Barry said.

The charter also stated that no one would be punished or "disquieted" for any difference in opinion in matter of religion, which Barry believes was a first.

"This was extraordinarily unusual, extraordinarily unusual," Barry said.

It would take generations for Williams’ views to be embraced as linchpins of the American experiment, largely due to his influence on English writers John Milton and John Locke, said Barry. Their works, in turn, would deeply guide the thinking of many of the founding fathers.

"He’s not just a precursor of the way a lot of people think today, but is emblematic of that thinking, and indeed I think some of the things he stood for would be considered extreme today," Barry said.

Roger Williams, believer in separation of church and state, advocate of religious liberty, founder of Rhode Island, was banished from Massachusetts, 380 years ago this week.

If you have a tale of forgotten Massachusetts history, let Edgar know. Email him at curiosity-desk-at-wgbh-dot-org.