One Sunday morning during Thomas Jefferson's presidency, a friend stopped him on his way to Christ Church, then meeting on Capitol Hill. The president had a prayer book tucked under his arm. The man was incredulous. "You do not believe a word in it," he said.

Jefferson, pilloried as the village atheist during his first presidential campaign, was unruffled. "Sir," he replied, "no nation has ever yet existed or been governed without religion. Nor can be." He explained that, as the nation's chief executive, he was obliged to give religion its public due.

The story is worth recalling as we mark the 200th anniversary of Jefferson's famous statement on religion and politics -- that the First Amendment built "a wall of separation between church and state." Many will celebrate the wall metaphor as the defining feature of America's secular republic. But recent scholarship into its background may temper the festivities.

The expression is found nowhere in the Constitution, appearing instead in a letter to Connecticut Baptists dated Jan. 1, 1802. It was largely unheralded, in fact, until the Supreme Court invoked it in a 1947 case concerning government aid to parochial schools. Courts now cite the phrase to deny any form of public support for religion. Liberals quote it as holy writ, while conservatives dismiss the letter as marginal to the religious temper of the Founders.

Historians are casting fresh doubts on each of these views. In a forthcoming book, "Thomas Jefferson and the Wall of Separation," American University Professor Daniel Dreisbach argues that the letter's meaning cannot be understood apart from the politics of the time. A culture war was raging: Jefferson's Republicans, jealously protective of the separation of powers, accused the Federalists of secretly being monarchists, keen to exploit religion for partisan purposes.

Jefferson meant to counter the smear campaign against him and his Republican principles. But he also intended to soothe his allies. "It was a political statement," Dreisbach writes, "carefully crafted to reassure Jefferson's Baptist constituents in New England of his continuing commitment to their religious rights." In much of colonial America, where Congregationalism or Anglicanism enjoyed tax support as state churches, Baptists struggled as a beleaguered minority.

Their resentment fueled the political quest for religious liberty. Jefferson agreed with these dissenters -- as did virtually all the Founders -- that when government coerces conscience in matters of faith it threatens both civic peace and the purity of religion. That's why he begins the letter by insisting that "religion is a matter which lies solely between Man and his God." It was the same argument he used 17 years earlier in a "Bill for Establishing Religious Freedom" in his native Virginia -- a landmark victory made possible by massive evangelical support.

Here was a view of the sanctity of conscience held as tenaciously by Baptist preacher John Leland as it was by Enlightenment leader John Locke. Jefferson's wall, Dreisbach concludes, wasn't meant to bar religion from public life but to prevent faith from being either politicized or tread upon by government.

James Hutson, manuscript curator for the Library of Congress, agrees. He says Jefferson placed great value on symbolic support to religion. Two days after writing the letter, the president attended church services in the House of Representatives, a practice he would continue for years. He opened up federal buildings to a variety of religious services. "It is no exaggeration to say that, on Sundays in Washington during Thomas Jefferson's presidency, the state became the church," Hutson says.

Jefferson was no closet Christian, nor did he approve of federal subsidies for churches. He notoriously declined to proclaim national days of thanksgiving. But government at all levels could accommodate religious expression -- even worship services -- as long as it was voluntary and the state didn't pick favorites. Jefferson saw no conflict between the First Amendment and the availability of public property, public facilities and even government personnel to religious bodies.

The reason went beyond mere politics. When Jefferson remarked that no nation could be governed without religion, he did not have in mind the corrupted variety of government churches. In this, he argued exactly as the most pious Founders did: Religious belief -- freely chosen and given wide public space -- nurtured morality and thus supported a free society.

That ought to make both the theocrats and the secularists uneasy. For Jefferson's wall between church and state was meant to serve a greater goal -- to promote and preserve religious liberty for Americans of all faiths.

Joseph Loconte is the William E. Simon fellow in religion and a free society at The Heritage Foundation, and a commentator for National Public Radio.