Each year as Independence Day approaches, John Adams’s famous letter of July 3, 1776, to his wife Abigail is recalled and widely quoted.

Adams was writing from Philadelphia, one day after the Continental Congress voted unanimously to declare independence from Great Britain. To his wife back home in Braintree, Mass., he overflowed with elation:

“The Second Day of July 1776, will be the most memorable Epoch in the History of America,” Adams exulted.

I am apt to believe that it will be celebrated, by succeeding Generations, as the great anniversary Festival. It ought to be commemorated, as the Day of Deliverance by solemn Acts of Devotion to God Almighty. It ought to be solemnized with Pomp and Parade, with Shews, Games, Sports, Guns, Bells, Bonfires and Illuminations from one End of this Continent to the other from this Time forward forever more.

As it turned out, of course, the “great anniversary Festival” would be celebrated not on July 2, the date of the vote for independence, but on July 4, the day when Congress approved the final text of the Declaration of Independence. The earlier date was the more logical choice, but when the declaration was engrossed on parchment for signing, the words “In CONGRESS, July 4, 1776” appeared prominently on top.

Adams was right about the “Pomp and Parade” and “Bonfires and Illuminations” that would be indelibly associated with Fourth of July festivities. The tradition of setting off fireworks began with the very first organized celebration of Independence Day on July 4, 1777. “The evening was closed with the ringing of bells,” reported Philadelphia’s Evening Post on July 5, “and at night there was a grand exhibition of fireworks … on the Commons, and the city was beautifully illuminated.”

But what about the “solemn Acts of Devotion to God Almighty” that Adams foresaw as a mainstay of the holiday? Those haven’t lasted as a July 4th tradition — a development that would likely have distressed not only Adams, a devout churchgoing Congregationalist, but perhaps even his less worshipful contemporaries, such as Thomas Jefferson. For the last thing any of the founders of the American republic wanted was a society in which religion would be disregarded, or in which references to God would be seen as unwelcome or out of place.

Jefferson, for example, though famously skeptical of much religious dogma, nevertheless believed in a benevolent creator God to whom humans owed gratitude. “The god who gave us life, gave us liberty at the same time,” he wrote in 1774, and the Declaration of Independence refers to God not once but four times. Its opening sentence invokes “the Laws of Nature and of Nature’s God.” Its final paragraph begins by “appealing to the Supreme Judge of the World for the Rectitude of our Intentions.” It closes “with a firm Reliance on the Protection of divine Providence.” And in the most quoted passage in our history, the Declaration of Independence puts God at the very heart of the American Idea:

We hold these Truths to be self-evident: that all Men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the Pursuit of Happiness.

In linking religion to American liberty, Adams and Jefferson were not simply bowing to the political correctness of their time, or verbalizing empty sentiment that no one was expected to take seriously. They were articulating a core principle of American nationhood: Religious faith — and the civic virtues it gives rise to — is as indispensable to a democratic republic as freedom of speech or the right to own property. Religion can survive in the absence of freedom. But freedom without religion, they knew, tends to become dangerous and unstable.

Over and over, the Founders said so. “Of all the dispositions and habits which lead to political prosperity,” George Washington reminded the country in his farewell address, “religion and morality are indispensable supports.”

Even Benjamin Franklin, arguably the most cosmopolitan and scientifically minded of the Founders, argued that American independence had to be grounded in religious faith. “I have lived, sir, a long time,” he said during the Philadelphia Convention of 1787, “and the longer I live, the more convincing proofs I see of this truth: that God governs in the affairs of men.” In his autobiography, Franklin wrote that in his younger years he became a “thorough deist” — i.e., someone who believes that God exists but plays no active role in the universe — yet that view clearly changed over time. Disturbed that the Philadelphia convention’s sessions weren’t being opened with prayer, he introduced a motion to begin doing so. In a private letter to Ezra Stiles, the president of Yale, Franklin summarized his religious creed:

I believe in one God, Creator of the Universe. That he governs it by his Providence. That he ought to be worshipped. That the most acceptable Service we render to him is doing good to his other Children. That the soul of Man is immortal, and will be treated with Justice in another Life respecting its Conduct in this. These I take to be the fundamental Principles of all sound Religion.

Many of those who were at the center of America’s struggle for independence were sure that it was being guided forward by more than their own mortal efforts. In an influential sermon in 1776, the Rev. John Witherspoon — James Madison’s teacher at Princeton and a leading member of the Continental Congress — argued that God’s hand could be discerned in the gathering storm and in the chain of events that had led to it. “It would be a criminal inattention,” he said, “not to observe the singular interposition of Providence hitherto, in behalf of the American colonies.”

At a very different moment 11 years later, reflecting on the remarkable unanimity achieved by the Constitutional Convention — a body that should have been riven by bickering factions — Madison likewise saw divine intervention.

“It is impossible,” he wrote in Federalist No. 37, “for the man of pious reflection not to perceive in it a finger of the Almighty hand which has been so frequently and signally extended to our relief in the critical stages of the revolution.”

Liberty with faith, a secular state nourished by a religious society — that was the formula the Founders devised. They sought to combine the Enlightenment’s emphasis on reason, learning, and pluralism with the Judeo-Christian ethic of responsibility, justice, and goodness. What resulted was a nation that proved to be, on the whole, more diverse, more free, more tolerant, more successful, and more religious than any before or since.

Religious attachment is not as strong in America as it used to be. According to Gallup, just 72% of Americans say that religion is important in their lives, down considerably from the 95% who said so in 1952. Might there be a connection between that decline and the acrimony, mistrust, and unhappiness that have grown so pervasive in American life?

The men and women of 1776 have long since gone to their reward, but there is enduring wisdom in their conviction that while liberty is the purpose of a just government, the survival of that liberty requires the steady cultivation of virtue. Self-government is the most difficult method by which human beings can organize a society, and to do so successfully depends on recognizing that each citizen is created in the image of God, and is entitled therefore to political autonomy and freedom of conscience. “The rights of man come not from the generosity of the state but from the hand of God,” declared John F. Kennedy in his inaugural address. That has always been a core American value.

Much has changed in the 244 years since John Adams wrote in jubilation to his beloved Abigail, but it remains our responsibility to pursue today what they and their contemporaries fought so long ago to achieve: one nation under God, with liberty and justice for all.

(Jeff Jacoby is a columnist for The Boston Globe).