We have embarked on yet another presidential campaign in which religion will play an important role without any agreement over what the ground rules for that engagement should be.

If you think we're talking past each other on jobs and budgets, consider the religious divide. One side says "separation of church and state" while the other speaks of "religion's legitimate role in the public square." Each camp then sees the question as closed and can get quite self-righteous in avoiding the other's claims.

Anyone who enters this terrain should thus do so with fear and trembling. But a few things ought to be clear, and let's start with this: The Mormon faith of Mitt Romney or Jon Huntsman should not be an issue in this campaign. Period.

In the United States, we have no religious tests for office. It's true that this constitutional provision does not prevent a voter from casting a ballot on any basis he or she wishes to use. Nonetheless, it's the right assumption for citizens in a pluralistic democracy.

All Americans ought to empathize with religious minorities because each of us is part of one. If Mormonism can be held against Romney and Huntsman, then everyone else's tradition -- and, for non-believers, their lack of religious affiliation -- can be held against them, too. We have gone down this road before. Recall the ugly controversy over Catholicism when Al Smith and John F. Kennedy sought the presidency. But to say this is not the same as saying that religion should be completely excluded from politics. The test should be: To what extent would a candidate's religious views affect what he or she might do in office?

Many beliefs rooted in a tradition (the Virgin birth, how an individual keeps kosher laws, precisely how someone conceives the afterlife) are not relevant in any direct way to how a candidate would govern. Yet there are many questions -- and not just abortion -- on which the ethical and moral commitments that arise from faith would have a direct impact on what candidates might do in office. Those should be argued about. This leads to a conclusion that philosopher Jean Bethke Elshtain reached some years ago: "Separation of church and state is one thing. Separation of religion and politics is something else altogether. Religion and politics flow back and forth in American civil society all the time -- always have, always will."

That is entirely true. It's also not as simple as it sounds. For if religious people fairly claim that faith has a legitimate place in public life, they must accept that the public (including journalists) is fully justified in probing how that faith might influence what they would do with political power.

Religious people cannot have it both ways: to assert that their faith really matters to their public engagement, and then to insist, when it's convenient, that religion is a matter about which no one has a right to ask questions. And here's the hardest part: We all have to ask ourselves whether what we claim to be hearing as the voice of faith (or of God) may in fact be nothing more than the voice of our ideology or political party. We should also ask whether candidates are merely exploiting religion to rally some part of electorate to their side. The difficulty of answering both questions -- given the human genius for rationalization -- might encourage a certain humility that comes hard to most of us, and perhaps most of all to people who write opinion columns.