The positive law codifies moral judgments the chief vehicle of which is religion. Attacks on religion therefore tend to undermine morality, and with it, the rule of law and respect for the rule of law. Is this thesis supportable?
Religion could be kept private and out of the public square. Many think that it should be. But law can't: the whole point of law is to set forth prescriptions and proscriptions for the behavior of citizens, especially in their relations to one another. The suggestion that religion be relegated to the private sphere is not incoherent. The suggestion that law be so relegated surely is.
Law, however, presupposes morality: the positive law is largely a codification of our moral judgments as to the permissible, the impermissible, and the obligatory. We have laws against drunk driving, for example, because of our antecedent conviction that it is morally wrong to act in ways that needlessly endanger ourselves and others. The legal rests on the moral: legal normativity is grounded in moral normativity. If a law is genuinely normative, then it derives that normativity from a sound moral basis; otherwise the law is merely the say-so of legislators and not deserving of respect, though it might be something to fear. But while the legal rests on the moral, it must not be conflated with it. One proof of this is that laws can be morally evaluated. Morally sane people have no trouble pronouncing certain laws immoral. One thinks of the Nuremberg Laws of the Third Reich. It would be no defense of these laws to insist that they were legal and enacted according to all the relevant protocols of the judicial system then and there in effect. 'Illegal law' is a contradition in terms. 'Immoral law' is not. The positive law is subject to moral evaluation.
Now the chief vehicle of morality for most people is and has been for centuries religion. The Ten Commandments, for example, are to be found in the Old Testament, and are at the ethical center of both Judaism and Christianity. There is much more to both religions than their respective ethical teachings, and there is much more to their ethical teachings than the Decalogue; but the latter is surely at the center of the ethical doctrine of both religions.
Attacks on religion, therefore, are indirect attacks on the morality of which religion is the vehicle, as well as indirect attacks on the law that codifies the morality.
Consider drunk driving again. It is illegal. And it ought to be: its legal impermissibility is morally defensible. To mount a moral defense of the law you have to argue from some moral principle or principles. Ultimately you will be arguing from "Thou shalt not kill," one of the Ten Commandments. Because we judge it immoral to take innocent human life, we also judge it immoral (thought not as immoral) to recklessly endanger human life as drunk drivers do.
"But couldn't one keep the moral prohibitions against killing, stealing, lying, etc. as a basis of public policy while relegating the religious 'packaging' to the private sphere?"
There are two questions here. The deeper one concerns the foundation of morality. To put it graphically, can the normativity of Judeo-Christian ethics survive the death of God? That may not be the best way to frame the question, but it conveys its flavor. My present concern, however, is with a less deep question: Does it make sense to attack the main means for the majority of people to acquire moral formation and guidance?
You know what my answer is.