Is it morally appropriate for citizens in a liberal democracy like the United States to support or oppose public policies solely for religious reasons? Although regularly serving as grist for the mill of political theorists, that question is not the familiar fare of ordinary political discussion. It's not a question about, or at least directly about, which laws our government ought to enforce. We're all too familiar with such questions—about the moral propriety and practical wisdom of abolishing the death penalty, legalizing abortion, declaring war on Afghanistan, and so on. Rather, it is a question about the kinds of justifications citizens should or should not have for their political commitments.

The most common position on this issue calls for a general constraint on the political use of religious reasons. Proponents of this constraint argue that citizens must support public policies for secular reasons...