When Mitt Romney delivers his talk about faith and America on Thursday, he will be writing a new chapter in one of the country's longest-running tales, the story of the tension between religion and liberty and church and state. Questions do not get any more fundamental than the one Romney has set out to address, for America was explicitly founded on the principle that religion, while a critical element in a republic, should not dominate that republic's political life.

The former Massachusetts governor is giving the speech because he is a Mormon, a member of the Church of Jesus Christ of Latter-Day Saints. There are conservative Christians (and no doubt others) who believe Mormons are not Christians, or that the church is a cult. A new study from Vanderbilt University's John Geer, released on Wednesday, has found that "bias against Mormons is significantly more intense among the public compared to bias against women and blacks." And it seems safe to say, too, that Romney would be spending Thursday very differently if Mike Huckabee were not ahead in the Iowa polls.

While the impetus may be tactical, Romney has an unusual opportunity to revisit some of the most compelling history in the American experience. He should say clearly in his speech that he will not allow his church to dictate to him on public matters, and that he will always explain himself if or when a specific political position he holds is linked to a doctrine of his faith: we deserve to know that much of any candidate. Beyond that, he should talk about how religion has shaped us without strangling us, and that the Founders envisioned a nation in which religion would be one factor among many in the life of the country. (An odd disclosure: it has been reported that Romney is reading "American Gospel," a book I wrote on this subject in 2006, in preparation for his address at the Bush Presidential Library in College Station, Texas.)

It is not an easy speech to give. The role of religion in politics tends to create extreme positions—or at least those who hold the more extreme positions are a good deal louder than more moderate voices. On the one hand there is a strong sense in the country that America is on the road to theocratic rule, that evangelical Christians are on the march and that the Founders were all about the "wall of separation" written about by Thomas Jefferson. On the other hand are many religious people who mistakenly think that America was founded as a "Christian nation" (which it was not), that the Founding Fathers were apostles in knee britches (which they were not) and that liberal activist judges have systematically stolen the country's religious heritage (which they have not).

Neither side has it right. The separation of church and state—including the explicit prohibition against a religious test for office in the Constitution—was essential to the Founders, but they also understood that religion and politics were always going to be mixed up together. The critical thing was to manage this human reality, to minimize its ill effects and to make the most of the possible good it could do. And so if Romney wishes to argue that religion is important but not all-important, and that judging candidates by sectarian labels is not what America was intended to be about, then history is on his side.

These questions are hardly new. In 1800 there were advertisements saying voters could have "Adams and God, or Jefferson and no God." Three decades later Andrew Jackson had to resist the formation of a "Christian Party in Politics." Abraham Lincoln buried a proposed constitutional amendment designed to declare the nation's dependence on, and allegiance to, Jesus. The only words FDR spoke in public on D-Day were those in a prayer of his composition, which he read over the radio to an audience of 100 million Americans, perhaps the largest mass prayer in human history. And the last line of the ur-text of modern liberalism, JFK's inaugural, was: "On earth, God's work must truly be our own."