03/08/2012 10:48 am ET Updated May 08, 2012

Rick Santorum and John F. Kennedy

Reacting to John F. Kennedy's famous 1960 speech about the role of religion in politics, Rick Santorum said this, on ABC's This Week: "I don't believe in an America where the separation of church and state is absolute. The idea that the church can have no influence or no involvement in the operation of the state is absolutely antithetical to the objectives and vision of our country." To understand why Santorum is partly right, and partly wrong, one must first take a closer look at Kennedy's speech.

The young senator from Massachusetts was seeking to quell concerns that as a Catholic he would be beholden to the pope. Kennedy did so by stating that the separation of church and state is absolute, that religious leaders shouldn't tell presidents how to act or parishioners how to vote, that religious schools should receive no public funds, and that public officials should neither request nor accept instructions on public policy from religious leaders or institutions.

Santorum is correct to think that Kennedy went too far. Religious leaders and institutions have a right to speak out on the issues of the day, including speaking to public officials. This right is based not only in the free exercise of religion but also in the freedom of speech. Moreover, after many twists and turns, the Supreme Court now clearly permits public funds to be channeled to religious schools -- so long as such funding programs (say, educational vouchers) are part of a general package that includes secular schools, and so long as the funds aren't earmarked for religious ends. Furthermore, although an unadorned nativity scene on a county's courthouse steps is unconstitutional, towns may celebrate the holiday season with an array of symbols, religious and secular. We do not have, and have never had, an absolute separation of church and state.

But Santorum is wrong to suggest that religious institutions may have "involvement in the operation of the state." In a little-known case called Larkin v. Grendel's Den, the Court invalidated a Massachusetts law that gave churches a veto right over liquor licenses for nearby establishments. Churches and church leaders may seek to influence public policy, but may have no formal role in making it.

The hardest question in this area is whether laws may be based in a predominantly religious purpose. The Court has clearly said No, striking down laws forbidding the teaching of evolution and requiring that creation science be taught if evolution is taught, and nixing a Ten Commandments display in a local courthouse. In each case, the Court's concern was that dominant religious groups had captured the political process for their doctrinal ends. Instead, laws must have predominantly secular bases, even if those bases will often overlap with religious ones.

How and when we separate church and state is a complex matter. Rick Santorum is right to press the issue as one of religious freedom. But while such freedom sometimes permits a role for religion in politics, other times it requires the opposite, a lawmaking process based in secular justifications, without formal church involvement.

Abner S. Greene is a professor at Fordham Law School, and the author of Against Obligation: The Multiple Sources of Authority in a Liberal Democracy (Harvard University Press 2012).