This necessity is important for two reasons. First, those principles all or virtually religions agree upon are important ones worthy of constant attention. These include that we live in a larger context than our private interests, that this context has powerful ethical dimensions and among these dimensions are values such as kindness, generosity, and a dislike of violence. Religious people putting their arguments in these terms not only enrich the national dialogue, they also become more aware of what they share in common with others. Given Christianity's long history of religious violence and sectarianism, this is important.
This brings me to the second reason. Having to put political arguments in broadly secular terms weakens sectarianism's poisonous influence. It rewards people in different faith communities that can communicate with other communities about issues of common concern. Democracy and freedom are strengthened.
People's religious commitments gave them the moral strength to stand for what they regarded as right while the need to appeal to people in other faith traditions forced them to argue in universal terms. To the degree their motives and their methods were harmonious, they contributed to raising the moral level of their society. The Quakers achieved this goal while opposing slavery. Martin Luther King made his strongest arguments in terms that atheists and non-Christians alike could support because of our common humanity. His religion gave him added strength to risk his life, but his arguments to the rest of us did not depend on our agreeing with his version of Christianity. Or any version.
This kind of religious influence was what the Founders had hoped would happen.
The Danger of Religious Tyranny
Today, religious fanatics and bigots like Rick Santorum or psychopathic opportunists seeking their backing, like Mitt Romney, are destroying this foundation for American liberty and democracy. Along with their incessant lying, which destroys political debate and the discussion of ideas on which democracy depends, they are also destroying the country by casting their arguments in very narrow terms, and then making those issues so central that compromise is impossible. Political discussion that can enlarge citizens' understanding is replaced by self-righteous shrieking as we see in the falsely named "pro-life" movement that spreads death and suffering for real people while unctuously puffing about their devotion to zygotes. That their motives have nothing to do with life and everything to do with control and domination have been revealed for all but the most blind to see by their recent assaults against contraception and violence against women legislation, even to the point of permitting doctors to lie and occasionally prohibiting them from telling their patients the truth.
The requirement for a secular context within which citizens could make their case to other citizens is rejected as "anti-religious" when in fact it is pro-religious. In its place, they would install rule by privileged groups the worst members of which would rise to the top as genuine spiritual scum, something that we have observed happening whenever religion and politics become indistinguishable. Oppressing other views comes naturally to such people. Some of Santorum's supporters have said as much, although others are more circumspect.
Either they rule us, or they seek to destroy the country. There is no third option. Under such circumstances the stakes of winning or losing rise to become unacceptably high. No one who values women's freedom, be they man or woman, can afford to let these people rule us. No one who has any faith other than the most irrational and mindless reading of the Bible can afford to let them rule us either. Nor can anyone who has no faith. History is very clear where these people go when they have the power, and the graves of millions testify to their methods.
For the moment, right-wing Catholics are united with right-wing Southern Baptists, right-wing Pentecostals, and right-wing Mormons in a wholesale assault on America's best traditions. Their political leaders, such as Santorum, have made it clear that forms of Christianity not like his are not really Christian. Given that they claim America is a "Christian" nation, it takes little imagination to realize what they would do to liberal Christians once they have consolidated power. Totalitarians of the right and left have always been clear as to what they would do, and their future victims have refused to believe them, until it was too late. Now we are facing a similar challenge.
The logic of their positions would turn them against one another after having gotten rid of the wrong sorts of liberal Christians, Pagans, and likely Jews. This is what Christianist totalitarians have always done since the Roman Empire. Today in America, it is as if the worst people from times that repeatedly drenched Europe in war and massacre have crawled like mindless zombies out from the mass graves of mutual religious slaughter to do it again. And most of these zombies are Republicans.