IN SEPTEMBER 1777, the dream of liberty seemed all but extinguished. In Delaware, on the haunted date of Sept. 11, George Washington had suffered a devastating defeat in the Battle of Brandywine, in which 200 Americans were killed, 500 were wounded and 400 captured.

Congress had shrunk from the 56 men who had signed the Declaration of Independence a little more than a year earlier to a mere 20, who had fled Philadelphia to escape advancing British forces. The small group gathered in York, Pa., to discuss whether to go on.

“The prospect is chilling, on every Side: Gloomy, dark, melancholly, and dispiriting,” John Adams of Massachusetts wrote.

Historians like to talk about broad social forces that drive events, but history often turns on the character of a single man.

Such was the case in York, where a zealous defender of freedom rose and helped stiffen the spines of his fellow politicians.

“If we despond, public confidence is destroyed, the people will no longer yield their support to a hopeless contest, and American liberty is no more. . . . Despondency becomes not the dignity of our cause, nor the character of those who are its supporters,” he said.

“We have proclaimed to the world our determination ‘to die free men, rather than live as slaves.’ We have appealed to Heaven for the justice of our cause, and in Heaven we have placed our trust. . . . Good tidings will soon arrive. We shall never be abandoned by Heaven while we act worthy of its aid and protection.”

Congress listened, and the man was right. A few weeks later, at Saratoga, N.Y., 5,800 British soldiers surrendered to American Gen. Horatio Gates. French recognition of America as an independent nation followed, along with crucial military aid that turned the tide of the war and led to the founding of what would prove to be the freest, richest, most powerful nation in history.

The man at York was John Adams’s cousin, Samuel Adams, whose fervent commitment to the American cause was key to its ultimate success.

Later, when John Adams served as a diplomat in France, everybody mistook him for the “famous” Adams — Samuel. Nowadays, of course, John is the celebrated one: the petted subject of a warm and glowing biography by David McCullough and HBO miniseries. Sam Adams, meanwhile, is best known today as the name of a beer.

Ira Stoll, managing editor of the late New York Sun, attempts to rectify that in a sprightly new biography, Sam Adams: A Life (Free Press, $28).

It’s easy to see why Sam has fallen out of fashion. By our secular standards, he was a religious zealot, a true believer, who saw America as a “Christian Sparta.” He perceived America’s fight for liberty not as a revolutionary act but as a conservative one — to preserve the freedoms obtained at great cost by his Puritan forebears, particularly the freedom to worship God in a way that stood at odds with England’s state religion.

He hated the Church of England, thought it smacked of “Popery” (Roman Catholicism, which he despised), and worried that a British bishop would be set over America, to weaken the independent, soul-searching spirit of the colonies’ citizens, so that the government could enslave a populace of numbed and pliant sheep.

Mr. Stoll recognizes this element of Adams’s passion, and artfully sets it into the context of the religious movements of the times.

He also shows why Adams was a different kind of religious zealot than the intolerant, oppressive ones we think of. He was a fierce champion of freedom of the press, and warned repeatedly of the threat to liberty posed by a powerful central government. He cautioned about the influence of special-interest money in politics, and urged the public to beware of popular political figures who used the language of America’s ideals but only wanted to gain power for themselves.

“It is not infrequent,” he wrote decades before the Revolution, “to hear men declaim loudly upon liberty, who, if we may judge by the whole tenor of their actions, mean nothing else by it than their own liberty — to oppress without control or the restraint of the laws all those who are weaker and poorer than themselves.”

He also warned that liberty was only possible in a society shaped by values of honesty and consideration for others. “Neither the wisest constitution nor the wisest laws will secure the liberty and happiness of a people whose manners are universally corrupt,” he wrote.

These are ideas still well worth mulling today.

The British well understood that Samuel Adams’s drive and courage were crucial to the American cause. When Gen. Thomas Gage, Britain’s military governor of Massachusetts, made a last-ditch attempt at reconciliation with the colonists in 1775, he offered His Majesty’s “most gracious pardon” to all of the rebels, with the exception of two: Samuel Adams (listed first) and John Hancock.

After the British suffered their catastrophic defeat at Saratoga, Adams steered through Congress a national “day of thanksgiving” to God.

Maybe on our Thanksgiving Day, we can include Sam Adams in our thanks — not just on our tables — for the freedoms that we still manage to enjoy, all these years later.

Edward Achorn is The Journal’s deputy editorial-pages editor ( eachorn@projo.com).