There is a picture of Western democracy that is often invoked in the current debates over the place of religion in a secular society. According to this picture, the role of the state is to protect the public space in which political decisions are taken, in which agreements are brokered and in which the social and economic life of the nation is peacefully conducted. This public space must be accessible to every citizen. Differences of race, of wealth and also of religion should all be discounted on entering the public space. Those who attempt to capture that space in the name of a religion are the enemies of civil government. It is a space whose ruling principle is the freedom of conscience, including the freedom of religion - the first freedom defined and offered by the US Constitution. From this first freedom others follow, including freedom of speech and opinion. The law has, as one of its duties, that of protecting this public space, and that means preventing people from invading or curtailing the freedoms that define it.
Something like this picture was adopted at the Enlightenment; so we think at least. In Britain, it led quickly to Catholic Emancipation and then to a public ethos of toleration, extended towards religious minorities. Soon, the advocates of toleration were pressing for toleration to be extended not merely to minority religions, but also to minority lifestyles, and even to alternative moralities, John Stuart Mill famously arguing in On Liberty for a clear distinction between that which can be forbidden and that which, while morally disapproved, must nevertheless be permitted by law.
Mill enunciated the central doctrine of his argument thus: "The only purpose for which power can rightfully be exercised over any member of a civilised community against his will is to prevent harm to others." He added, "His own good either physical or moral is not a sufficient warrant. He cannot rightfully be compelled to do or forebear because it will be better for him to do so, because it will make him happier, because in the opinions of others, to do so would be wise or even right."
Freedom is vital for the health of a modern society, in Mill's view, since it is the sole means whereby new truths can be discovered, new forms of life embarked on, new consensuses established - in other words, it is necessary if societies are to adapt. And what does not adapt does not survive. To that utilitarian argument he adds, or at least hints at, another, which is that we have no right to forbid what does no harm to others. The right here is not just a legal right, obviously, since that would simply make the argument circular. Mill seems to think that there is no moral right to forbid what harms no one except the agent. That is because we have a right to freedom, a right which either lies in the nature of things or is fundamental to the kind of citizenship that we enjoy.