I’ll add another wrinkle to your question: how is that the least religious colonies of the 18th century – those of the South, where church attendance was markedly lower than in New England or the Middle Colonies – have now become the Bible Belt of the US?

One answer has been provided by Christine Leigh Heyrman in her book, The Southern Cross. Heyrman argues that evangelical churches struggled in the 18th century to gain inroads in the South, in large part because their religious message challenged the social hierarchy of the region: they preached racial equality (and even opposed slavery), emphasized religious bonds over familial bonds (which threatened the power of plantation patriarchs), and encouraged the participation of women (which did the same).

In the 19th century, though, these evangelical churches repackaged their message. They toned down their criticism of slavery, promoted a muscular version of Christianity that appealed to men, restricted the participation of women and African-Americans in church affairs, and emphasized public religious behavior – while reassuring men that they still ruled the private sphere. With this style of religion reinforcing the social order – rather than challenging it – evangelical churches grew by leaps and bounds in the South, ultimately turning it into the Bible Belt of the country by the 20th century.

Which returns us to your original question. I would challenge the notion that the Puritans were “pretty far right” on the political spectrum. At the core of the mission of Massachusetts was the notion that all colonists were “knit together in this work as one man” – meaning that they had a strong sense of mutual responsibility, which would probably be construed as socialist in modern terms (if we’re going to indulge in anachronism). Everyone was expected to look after their neighbors, and there was a strong sense of mutual responsibility.

By contrast, the Southern colonies had a more individualistic, live-and-let-live ethos. Every plantation owner was the king of his own castle, and there was relatively little support for state intrusion in family affairs. (See Mary Beth Norton’s Founding Mothers and Fathers on this point.) In the 18th and 19th centuries, the South had lower tax rates than New England, no support for public schools (which were strongly supported in the Northeast), and little support for investments in infrastructure. (A lot of this had to do with slavery – see Robin Einhorn’s American Taxation, American Slavery.) The style of religion that took hold in the South in the 19th century reflected this outlook, which helps to explain why this region still tends to lean right even today.

That's a fantastic question. I've also wondered about this myself. I'm very interested in how the Puritans, once the MOST conservative religion in Western countries, became the MOST liberal religions: Unitarians and Congregationalists.

The lesson here is that tribes of people and their practices and cultures can change drastically over time.

First, your can't apply our modern political spectrum to the Presbyterians and Independents who made up the early Puritan population in the Bay colony. [opinion: using these charged modern terms harm the understanding more than they help] We have to understand them contextually to their time - just as we don't judge Planck and Galileo side-by-side in the scientific realm, the Christian world post-Luther - and under Jacobethean Anglican rule - is the starting point for the puritan factions. It seems that American history and European history are segregated in such a way to mask this movement as a fluid movement between the English and New England puritan movements. Later, the very origins of modern liberty are debated in A.S.P Woodhouse's compilation Puritanism and Liberty.

While this is not comprehensive, the second part of your question is fairly straightforward to answer. As the puritans developed churches, these churches were Congregational in organization. In these churches the members of the church could actually vote to approve church doctrine. So the germ of ongoing reform was baked into the original puritan church structures. As time went on the churches split into several factions, the most "left" of which is Unitarian. In fact the original Puritan church in the Bay colony governed by Congregational polity - the First Church in Salem - is today a Unitarian Church. This is the same church that - nearly 400 years ago - banished Roger Williams to Rhode Island, as his view on baptism were veering close to heretical.

Sources for further reading:

Francis Bremer's Winthrop and The Puritan Experiment

Woodard's American Nations

Barry's Roger Williams and the Creation of the American Soul

A great example of the puritan focus on liberty - all 98 liberties, as outlined in the Massachusetts Body of Liberties This was adopted by the General Court in 1641.

disclaimer: I am not a trained historian, but I felt providing some of the lesser known foundations and institutions is a helpful addition. If the mods will leave this up, I will try and flesh it out better tonight!

As other have mentioned, the Puritans of Today are not the Puritans from yesterday, but more importantly, they were vastly outnumbered by successive waves of immigration.

If you look at the demographics of Massachusetts Today for instance, you'll see that over 50% of the population is catholic due to the vast numbers of Irish and Italians immigrating.

Since the secularization of New England is a relatively recent phenomenon, by the time it got going, the puritans had been irrelevant for quite some time. To the point where a better question would by to ask why a region that was so catholic became the least religious.

You're defining "right" as being what, religiously conservative?

The Puritans were radicals against the state. You could consider that left wing, you could consider that right wing. Yes, they were religious but they were also far more libertarian (in the sense of how much power they wanted the state to have) than almost anyone on the American political spectrum today.

It's also extremely hard to apply notions of left and right to people who lived a) before the French Revolution and b) before Marx.

There are a couple of reasons, the biggest of which was demographic.

You see, by the time the Puritan Migration ended around 1640, Puritans made up nearly the entire white population of North America. But within a century, so many more Europeans had migrated to North America that, even in Puritan colonies, the Puritans and their descendants were no longer the majority. Suddenly even the Puritans are okay with tolerance and religious pluralism, up to a point.

But the idea of indirect religious rule (preachers give orders to the government, which is expected to comply) had already gone the way of all things, had already been written off as a failed experiment, by the early 1630s. That's what the famous Antinomian Heresy Trials of 1640 were about: the last boatload of Puritans out of England found out that churches were being licensed and regulated by the government, not the other way around, just like everywhere else in the world, even in parts of Massachusetts with unanimous Puritan population. They rebelled, declared that the church was superior to the state, founded their own true-Puritan unlicensed church ... and got convicted of lawless anarchism (or as it was called at the time, antinomianism) and were permanently expelled from the colony.

Puritan rule, in the sense that most people think of it, lasted less than two decades, and had been over for more than a century by the time of the American revolution.

Others have covered well the point that Puritan emphasis on the "common good/communal salvation" shaped the later political development of Puritan areas. I'll offer some background on the conditions within these colonies themselves.

The "Puritan" heartland was not co-extensive with New England or the colonies established by Puritan settlers. The Plymouth Colony was settled primarily by "Separatists", a distinct sub-sect of English Puritans, but with a substantial amount of "Strangers" - settlers not animated by the Puritan religion. The Massachusetts Bay Colony was the most substantial Puritan colony and received the brunt of its Puritan inhabitants through the Great Migration from 1625-1640, where the colony grew to 15-20,000 inhabitants. Despite their similar religious identities, the Plymouth and Mass Bay colonies were notorious for their internal squabbles and remained as distinct political and religious entities until the British crown instituted the short-lived "Dominion of New England". By 1692, the two had been united - but the effects of 70 years worth of separation are still visible in some ways in Massachusetts.

The other major Puritan settlement existed along the coast of Connecticut and, particularly, along the fertile Connecticut River valley. Modern-day Connecticut was initially composed of the New Haven, Connecticut River, and Saybrook colonies that only later merged to form a united government. Western Massachusetts, Vermont, New Hampshire, and Maine were only settled at later dates. Though the settlers were, in large part, Puritan emigrants from the coast, they were joined by many distinctly non-Puritan (mostly Presbyterian) groups like Scots-Irish in southern NH so the religious aspect of these areas' governments was subsumed by civil institutions. Rhode Island is an outlier - founded by a settler expelled from Massachusetts Bay for heterodoxy and developing as a refuge for Quakers, Baptists, and other groups scorned by the initial Puritan settlers.

Puritans would be pretty far to the right on our modern political spectrum, and yet the states where they settled the most (Massachusetts in particular) are now solidly blue states and widely regarded as among the farthest left in the country

I won't re-hash some of the sentiments below, but I will provide some of the historical background to Puritan-inflected governance in New England. The Puritans were, by and large, a middle-class group that endorsed notions of local democracy channeled through the church. Municipal governance in Puritan colonies existed in unison with the Church - only the citizens of a town could vote in local matters and citizenship required church membership. The meeting-houses doubled as the civic institutions. Yet these governments also undertook civic matters - though religion played a substantial role in crafting society, Puritan settlers also brought with them a defined notion of governance. The civic responsibilities of the church remained even after downturns in religiosity and were eventually incorporated into civic institutions separate from the church.

The Puritans were not just a group of religious zealots (but they were.....pretty fucking zealous), they carried with them ideas about how to organize the political life of a society (as opposed to extraction-based or proprietary colonies in North America). A lot of those political sentiments remain in New England, so the question of "Why the Change?" isn't the best way to phrase it - there's a valid argument that the States with Puritan heritage haven't changed drastically in its sentiments and institutions.

The more narrow question of the "why/how did these settlers' descendants become less religious" is a bit harder to answer. Massachusetts (where the majority of the population of New England lived) was densely settled and had only limited fertile soil (really, only the Con. River Valley). A robust maritime economy developed from these conditions. Maritime colonies don't have the spatial needs of agricultural colonies; proper cities developed along the coast of Massachusetts and these cities eventually became too large to administer through solely church governance. The method of government (citizen democracy, lack of hierarchy) was simply shifted from the church to the "town" in these areas and with continued trading successes, many of these areas lost the harshness of Puritanism. As Puritan churches were rigidly anti-hierarchical, there was no macro-level countervailing influence to coerce residents of other towns back to rigid observation of Puritan code. Each town, more or less, made decisions for themselves.

The religious sentiment certainly still remained in smaller communities, which were notoriously fearful of "outsiders" - the Salem witch trials stemmed from accusations made, not by Salem town residents, but from an outer-lying village within Salem's formerly large municipal boundaries and are a good example of this fear developing into paranoia. New England would under-go various "Awakenings" over the next few centuries, areas settled by New Englanders were generally described as religious and conservative. Upstate New York was notorious in this regard, spawning many religious sects - most famously: Mormonism. Elements of Puritanism, like the proverbial "stuffiness" of New Englanders, intolerance of other religions within municipal boundaries, resentment of ostentatiousness marked New England for a long time, but more and more those qualities became cultural, not religious, attributes. By the mid 18th century, the most densely settled areas of Eastern Massachusetts were heavily involved in global trade networks, were populated by many descendants of Puritans for whom the idea of creating a "New Jerusalem" was less important, and were marked by capable civic institutions not reliant on the church.