WASHINGTON—The recent campaign forum at Saddleback Church in which U.S. presidential hopefuls Barack Obama and John McCain answered questions from pastor Rick Warren, the new star of the evangelical Christian movement, could only have happened in America. The rest of the world watched in wonder as the two candidates proclaimed their faith before the tribunal of God, trying to persuade the jury that their views on abortion, gay marriage, stem cell research, state-sponsored altruism and other “value” issues do not depart (too much) from dogma.

Not that the conduct of Warren or his followers was in any way intolerant. Quite the contrary: Both the host and the audience could not have been more congenial and fair-minded. But the forum highlighted the test that McCain, a conservative mistrusted by many conservatives, and Obama, a believer who some conservatives see as a closet Muslim, are having to pass with the evangelical community, an important factor in American politics.

What was fascinating about the event was not the questions or the answers—or even, as many commentators are pointing out, the stylistic differences between a Socratic Obama and a martial McCain. It was what the forum told the rest of the world about America.

The encounter at Saddleback, a megachurch in California’s Orange County, brought home to an international audience a great theme of American history: the tension between the theocratic and the secular. It has been there from the nation’s beginning, in the difference between the original Virginia settlers, whose ambition was not subordinate to religion, and the Pilgrims from the Mayflower, who wanted to establish the kingdom of God. And historians have pointed to the Great Awakening, the religious revival in 18th-century America, as the matrix from which the revolution sprang: British historian Paul Johnson sees in the fact that the American Revolution was “a religious event” the essential difference with the French Revolution. And yet the U.S. Constitution is a secular document that pretty much ignores religion until the First Amendment—which then lays in stone the separation of church and state.

In modern times, there were periods—the counterculture of the 1960s, Roe v. Wade in 1973—when the secular impulse had the upper hand. They were followed by the theocratic backwash—the evangelical revival that started in the 1980s and with which one in four Americans strongly identifies today.

This unresolved struggle—the 800-pound gorilla at the Saddleback forum—is what stands in the way of the Republican Party becoming truly the party of small government. The tension between theocrats and secularists in society at large is mirrored in the soul of Republican conservatives who are torn between individual liberty—their belief in the free ownership of guns or their aversion to high taxes—and the “values state”—their belief in making the government an agent of the “right” moral values.

There are times when the secular overshadows the theocratic among conservative Republicans. In such times, as happened with Barry Goldwater in the 1960s, the Republican Party produces leaders inclined toward a libertarian, small-government society. But there are times when the theocratic pulsation is stronger. That helps explain George W. Bush and the decisive growth of government under his presidency. Individual freedom and moral engineering are the systole and the diastole of the Republican heart.

Similarly dramatic is the contradiction among Democrats. Although they have made concessions to conservatives out of political necessity, Obama’s conversation with Rick Warren reminded us that the Democratic Party does not want the government to impose values on society through coercion. That is a secular, small-government belief. But the faith in government as the agent of social justice in everything from poverty alleviation to energy policy to the unionization of workers is a recipe for big government. It is also a theocratic proposition of sorts: the government as the agent of good.

When, if ever, will the tension between the secular and the theocratic, that is, between small and big government, be dissipated once and for all in American politics? Will it require a third party or will the change come from within one of the two parties?

I suspect the odds of the solution to this conflict coming from within the parties are stronger. If nothing else, the Saddleback event was illuminating in that it brought these underlying truths to the surface.