Bewigged and in breeches, stiff and stern — this is how we often picture our Founding Fathers. Politicians today, especially on the right, tap into this severe image of moral rectitude. They invoke America’s Christian heritage as a sacred touchstone, bequeathed to us by great men who cribbed from the Bible as they drafted the Declaration of Independence and the Constitution.

The founders are surely spinning in their graves at this gross distortion of history. Reverend Gary Kowalski, of Burlington’s Unitarian Universalist Society, sets the record straight in Revolutionary Spirits: The Enlightened Faith of America’s Founding Fathers. He paints lively portraits of six key figures — Ben Franklin, George Washington, Thomas Paine, John Adams, Thomas Jefferson and James Madison — tracing their relationships to organized religion and, as much as possible, their personal spiritual beliefs and practices.

In examining the faith of the Fathers, Kowalski finds no Bible-thumpers in the bunch. Separate spiritual paths took all six to roughly the same philosophical place: a free-thinking Deism, approaching God “through the phenomena of nature,” Kowalski writes. “The cosmos presented a folio of order and regularity signed in the Creator’s own hand, a testament prior to any written scripture.” The Fathers also arrived at the common political goal of completely separating church and state authority.

Kowalski precedes the biographical sketches with a bracing overview of 18th-century America’s “religious mélange.” Many colonists had come from Europe as religious dissenters or refugees, contributing to the diversity of faiths and multiplicity of sects. But others “were not particularly pious at all.” In fact, the English government had actively encouraged not only “religious troublemakers” but also “other undesirables” to depart for the colonies: “the destitute and felons . . . entrepreneurs, adventurers, and vagabonds.” As a result, just one in eight colonists belonged to a church at the time of the American Revolution.

Kowalski links the colonists’ rebellious streak to their “unchurched” status. Those “not eager to submit to any ecclesiastical body that might restrict their personal liberty” increasingly chafed at England’s outrages. But diversity and fractiousness also presented the founders with a stiff challenge: bringing together headstrong countrymen “of many faiths and no faith . . . into a union of shared aspirations and commonly held values.”

The founders themselves were men of faith. Kowalski makes thoughtful connections between their religious roots and political perspectives. Paine’s Quaker background gave him “a commitment to radical equality and a fierce antipathy to privilege.” Adams’ Calvinist upbringing, with its heavy emphasis on original sin and man’s fallen state, colored his view on the necessity of constitutional checks and balances. “My fundamental maxim of government is, never to trust the lamb to the custody of the wolf,” Adams wrote. Madison, part of Virginia’s Anglican aristocracy, saw how the colony’s established church corrupted both pulpit and politics.

Although most of the founders drifted away from regular churchgoing and identifying with a particular sect, they shared a restless curiosity about questions that doctrine and Scripture failed to answer. They read and wrote widely on religion, philosophy and especially science, which fascinated them.

Kowalski provides delightful examples of how the amateur scientists mulled over theories and tinkered with inventions. Adams mused about intelligent life on other planets; Washington devised a seed-drilling plow. Jefferson and Madison devoured the works of a French proto-evolutionary scientist and created their own studies. Jefferson compiled a survey of indigenous North American quadrupeds, “a model of zoological thoroughness . . . from the tapir to the caribou,” Kowalski notes. Madison dissected weasels, sending the specimens to Jefferson in Paris, to compare the morphology of small mammals on different continents.

A scientific worldview underpinned the political philosophy that the founders embraced. John Locke advocated freedom of religion as part of his belief that government had no business interfering in private matters. Voltaire based his view on a pragmatic look at history: “The existence of only one religion in a nation produces slavery and two ignites civil war, while a multitude produces peace.” Madison looked at 15 centuries of established Christianity. “What have been its fruits?” he asked rhetorically. “Superstition, bigotry, and persecution.”

The quiet Madison became the firmest advocate for free exercise of religion. No compromise would suffice. In recounting the founders’ ultimate success in enshrining this right, Kowalski only hints at the ferocity of the opposition, briefly mentioning other options on the table (tolerance; absence of religious tests) and the long road to extending religious freedom to all states. But he does capture something essential: how and why the founders arrived at their convictions.

Kowalski’s crisp tour through this feisty period of American history charmingly distills the founders’ vigor and vitality. Intellectually audacious and politically brave, these men of faith believed that uncoupling religion and politics would ultimately increase virtue in both the public and private spheres. In the country’s current season of overheated campaign rhetoric, Kowalski’s book is a timely reminder of what the founders would think about remixing the two forces they fought so hard to separate.