Skepticism about the Christmas story is nothing new. In the 1790s, the famous political theorist, Thomas Paine, caused a fury when he called the doctrine of the virgin birth "blasphemously obscene." What is new, however, is the widespread effort to either publicly silence or sanitize the essentially religious message of Christianity. Crosses, creches, carols - all are being challenged in the courts as never before. Jay Leno even joked that there's a move afoot to rename the classic Macy's Santa movie to "Coincidence at 34th Street."

Yes, secularists and others are right to argue that government has no business openly endorsing religion. They're right, too, when they point to the separation of church and state as part of the genius of American democracy. It's the most important reason why the United States, a nation of staggering religious diversity, has avoided the sectarian violence that plagues so much of the rest of the world.

Indeed, there's something just a little unseemly about evangelical Christians going to court to keep nativity scenes planted in front of public buildings. The Alliance Defense Fund's "Christmas Project" is mobilizing no fewer than 700 legal advocates "to combat any attempts to censor the celebration of Christmas" in schools and on public property. There's a new twist on the manger scene: Mary, Joseph, Jesus, shepherds, wise men - and an army of lawyers.

Nevertheless, opponents of public references to Christianity are badly mistaken in their assumptions and their aims. Their campaign to secularize the public square ignores the powerful influence of Christian belief on American civic and political life.

Liberal civil-rights groups talk as if America's Founders were brooding secularists, at best indifferent to Christianity. The frivolous and relentless litigation of these groups has created a cloud of ignorance and intimidation, especially in public schools. Earlier this month in Cupertino, Calif., for example, a school district reprimanded a fifth-grade teacher for exposing his students to statements about God and the Bible from famous political leaders and documents. One watchdog group assailed the teacher and his supporters as "theocratic-loving" fanatics.

The problem is not would-be theocrats. The problem is the growing number of citizens who know next to nothing about the contribution of faith to democratic government. Students hear about Thomas Jefferson's "wall of separation" between church and state, but never learn that during his presidency church services were held in the U.S. Treasury, the Congress, and inside the Supreme Court chambers. As James Hutson, chief of the manuscript division of the Library of Congress, summarizes it: "It's no exaggeration to say that, on Sundays in Washington during Thomas Jefferson's presidency, the state became the church."

Although unorthodox in his own beliefs, Jefferson was typical of the Founders in this sense: He considered religion, especially Christianity, essential to democracy. The consensus of America's greatest political generation was that self-government required citizens of virtue, and virtue depended largely on religious belief. James Madison extolled Christianity as a "precious gift" to the young nation, while George Washington considered religion and morality "indispensable supports" to republican government. John Adams used his inaugural address to remind Americans that "a decent respect for Christianity [was] among the best recommendations for public service."

Many Americans, it seems, have lost that "decent respect" for the Christian faith. Some for example, regard Kwanzaa as culturally important as Christmas. But think about it: How does a black identity movement created in 1966 by the radical activist Maulana Karenga - jailed for torturing two of his female disciples - stack up to the religion of Jesus Christ? Others, lamenting the political clout of religious conservatives, compare devout Christians to Islamic terrorists. As author Gary Wills warned recently, we should "fear jihad, no matter whose zeal is being expressed."

To disdain a zeal for Christian truth, however, is to reject much of what has made the United States a decent and democratic society. The fight to end slavery, the protests against child labor, the early literacy campaigns, the army of anti-poverty groups, the Civil Rights movement - all owe their lifeblood to Christian conviction. The language of Christianity, writes historian William May, is "the language in which most Americans, during most of American history, did their thinking about human nature and destiny."

This leads us back to the Christmas story which, in its essence, is about the redemption of human nature and human destiny. Other religions, such as Buddhism and perhaps Islam, can manage pretty well without miracles and divine grace. Not Christianity.

For at the heart of the Christian faith is the Incarnation, "the grand miracle," as C.S. Lewis once put it: the claim that the Eternal God became mortal, vulnerable - even to the point of death - in order to rescue men and women from their guilt and shame. The scientist cannot accept the virgin birth of Jesus, but the deeper scandal to the modern mind is not just the nature of the Incarnation, but its purpose. "You are to give him the name Jesus," the gospel writers declare, "because he will save his people from their sins" (Matthew 1:21). Or as Lewis captured it: "The Son of God became a man to enable men to become sons of God."

Christians have always admitted this to be an almost incredible idea. The apostle Paul called it a profound mystery. The early church struggled to articulate it through its creeds. Protestant Reformer Martin Luther, with his German wit, said the Incarnation contains three miracles: "The first, that God became man; the second, that a virgin was a mother; and the third, that the heart of man should believe this."

A democratic state is not, of course, required to believe. But the story of this particular democracy cannot be understood apart from this narrative of faith. Government must not "establish" this religion (or any other), but neither should it banish any hint of Christianity's influence over our common public lives. At the very least, our leaders owe it their public respect.

An earlier generation of Americans understood the value of public reverence. "Here, at home, we will celebrate this Christmas Day in our traditional American way because of its deep spiritual meaning to us," President Franklin Roosevelt told the nation on Christmas Eve, 1944. "Because the teachings of Christ are fundamental in our lives; and because we want our youngest generation to grow up knowing the significance of this tradition and the story of the coming of the immortal Prince of Peace."

If that sentiment amounts to religious zeal, perhaps America would be a stronger and more just society with more of it, not less.

Mr. Loconte is a fellow at the Heritage Foundation and editor of "The End of Illusions: Religious Leaders Confront Hitler's Gathering Storm" (Rowman & Littlefield).