“These are the times that try men’s souls. The summer soldier and the sunshine patriot will, in this crisis, shrink from the service of their country; but he that stands it now, deserves the love and thanks of man and woman. Tyranny, like hell, is not easily conquered; yet we have this consolation with us, that the harder the conflict, the more glorious the triumph.”

These lines from Thomas Paine’s 1776 essay “The American Crisis” have resonated through American history. John Kerry’s participation in the “Winter Soldier” hearings that smeared American troops serving in Vietnam gave rise to the charge in the 2004 campaign that Mr. Kerry was a summer soldier himself. “These are the times that try men’s souls” has a rhythm to it such that I remember to this day studying the sentence on a blackboard in a middle-school English class decades ago, centuries after it was written. In January of this year, Norman Podhoretz began an essay in Commentary magazine on “The Panic Over Iraq” by quoting Paine’s lines.

The amazing thing is that “The American Crisis,” masterful as it is, wasn’t even Paine’s most significant essay. That honor belongs to “Common Sense,” the pamphlet that Benjamin Franklin said was published “with great effect on the minds of people at the beginning of the revolution.” Paine’s tombstone in New Rochelle New York, at his request, read, “Author of ‘Common Sense.'”

Paine himself lived a fascinating life, well told in Craig Nelson’s new biography, “Thomas Paine” (Viking, 396 pages, $27.95). After arriving in America from England in 1774 at age 37 and contributing with his pen to the American Revolution, Paine fetched up in France. There, though he could neither read nor speak French, he was made an honorary citizen and named one of a committee of nine that would draft a new French Constitution.

After falling out of favor with the French revolutionaries for his opposition to the execution of Louis XVI, Paine was thrown into a Paris jail for 10 months, during which he narrowly escaped both the guillotine and a bout with typhus. He spent the next 18 months residing with the American ambassador in France, James Monroe, then returned to America, where he was less than wildly popular, in part because of his writings from Europe against Christianity.

Mr. Nelson captures the arc of Paine’s life in lucid prose worthy at times of Paine himself. Mr. Nelson keeps his own political convictions mostly under wraps until the end of the book, at which point he claims “While Franklin, Paine, and Jefferson would be crestfallen that the modern-day federal government is the reserve of a new aristocracy — multimillionaire plutocrats and their corporate sponsors — Adams and Hamilton would be just as shocked to learn that their admired ruling elite no longer even pretends to lives of virtue.”

But there are more subtle shadings, too, even in this mostly straightforward and compelling account. Mr. Nelson makes much of Paine’s assertion in “The Age of Reason” that the Bible is not the word of God, and quotes Paine’s “The Age of Reason”: “I do not believe in the creed professed by the Jewish church, by the Roman church, by the Greek church, by the Turkish church, by the Protestant church, not by any church that I know of. My own mind is my own church.”

He spends less time explaining how Paine’s greatest achievement, “Common Sense,” connected with the American people in part by appealing to the Protestant anti-Catholicism that was common during the revolutionary era. Paine went on for pages quoting from the Bible, for example I Samuel, chapter 12: “We have added to all our sins this evil, to ask for ourselves a king.”

“Common Sense” summed it up: These portions of scripture are direct and positive. They admit of no equivocal construction. That the Almighty hath here entered his protest against monarchial government is true, or the scripture is false. And a man hath good reason to believe that there is as much of king-craft, as priest-craft in withholding the scripture from the public in Popish countries. For monarchy in every instance is the Popery of government.

Wrote Paine: “[W]hen a man seriously reflects on the idolatrous homage which is paid to the persons of kings, he need not wonder, that the Almighty, ever jealous of his honour, should disapprove of a form of government which so impiously invades the prerogative of heaven.”

None of these passages makes it into Mr. Nelson’s treatment of Paine. But surely the appeal to a higher authority is as much a part of why Paine is still relevant to today’s politics as was his skepticism of organized religion, which he grew more outspoken about only in the period after his greatest popularity and influence in America. Many see that skepticism as a reason to turn to Paine again in these new times that try men’s souls, but I see in “Common Sense” the news that even the most religiously unorthodox of the American revolutionaries invoked the Almighty in arguing for the justice of their cause.