The Rev. Rick Warren, the Billy Graham of the 21st century, gave his inaugural invocation on Jan. 20. President Barack Obama's invitation to Warren was controversial, but he won this election partly because he reached out to evangelicals like Warren and his followers. Obama campaigned to end our polarizing politics and unify the nation.

Some liberals are livid that Obama could pick someone so opposed to many of their causes. Warren is no Pat Robertson, but he opposes gay marriage and abortion rights. Obama campaigned for unity, but he also proclaimed the need for change, "change we can believe in." Isn't ending the strident influence of Christian conservatives a necessary change in post-Bush political landscape?

Advocates of the separation of religion and politics might wonder if we should have any religious voices so prominently featured in the Inauguration of the president. Paradoxically, the American tradition both invites and restrains religion in the public life of the nation. The United States is vigilant about both maintaining a secular state — and guaranteeing the free exercise of religion.

An uneasy coexistence
The Constitution forbids using religious identity as a prerequisite for holding public office.  However, the ban on religious litmus tests coexists — sometimes uneasily — with the free exercise clause of the Bill of Rights. Ardent pleas to separate church and state are juxtaposed with the religious politics of presidential candidates. Sen. John McCain called Jerry Falwell an "agent of intolerance" in the 2000 campaign — but later sought his support. Barack Obama titled his book after a sermon by his pastor, the Rev. Jeremiah Wright. But he rejected him before the fall election.

The challenge for President Obama — and any political leader — is to appeal to the followers of particular traditions in the context of a pluralist America. A president must preside over the entire country — not religious America or nonreligious America — but the entire country.

America is greatest when Muslims and Jews and Christians and nonbelievers live side by side in ways that are impossible in other parts of the world. In this land of liberty, people of diverse faiths usually gather not to condemn each other, but to do other things: to watch their children play soccer or hockey and maybe complain about their taxes.

Yet religion can threaten liberty. Pro-life citizens seek to restrain the liberty of women. In these United States, women are protected by a constitutional structure that separates government power and provides checks and balances. In this case, the Supreme Court has checked the power of state legislatures to criminalize abortion.  For pro-life citizens, they are free to organize.  Due largely to pro-life pressure, there are now four justices on the Supreme Court ready to overturn Roe v. Wade.

Moderating forces
The moderating forces of presidential politics provide another protection. Obama's biggest religious controversy was the extremist jeremiad of his own minister who preached sermons imploring God not to bless America but to damn it. Obama prevailed in the end largely because he sounded so unlike Wright. Rather than condemning America, Obama extolled a land of hope and opportunity.

Similarly, the Christian conservative pro-life movement is constrained by its marginal status.   When Republicans embrace the movement too strongly, they lose elections. Illustrating the tenuous influence of pro-life activists, three Republican-appointed members of the Supreme Court likely would not support overthrowing Roe v. Wade.

Amid these contending political forces, what principles guide our mix of religion and politics? During the Civil War, Abraham Lincoln wrote that both sides claim to act "in accordance with the will of God. Both may be, and one must be wrong. God cannot be for and against the same thing at the same time. In the present civil war it is quite possible that God's purpose is something different from the purpose of either party." Obama's calls for unity are unlike Lincoln's Civil War for the Union, yet Lincoln's theological humility is still apt.

In that spirit, there were many voices in the Obama inauguration. Warren gave the invocation, but who knows God's purpose?

Dan Hofrenning is a professor of political science at St. Olaf College in Northfield, Minn., and the author of "In Washington but Not of It: Religious Lobbyists Challenge the State."