This first part is of a general nature: It's about the whole idea of civil law and why it's important. Part 2 will talk about the applications of this idea to specific political issues.
A great number of human customs arose among people living in small societies that didn't travel much and didn't deal with other societies much, except occasional trade and occasional violence. It worked, as long as everyone involved was farming or otherwise tied down to the land in an area, and there wasn't much travel going on. The past several centuries have changed this, of course; people live in cities now more than in the countryside, and even in the most remote areas travel has increased, people interact with people all the time, and thanks to the new media, everyone's really in contact with a "worldwide civilization."
The enormous difficulty this creates is that groups with very different ideas of how life should be lived are now forced to live next door to one another. The "traditional way" of dealing with this is that one group would be dominant, and everyone else would live essentially by the graces of that group; different people might have power in different places, and whenever the power switched (like in Spain during the Reconquista) the members of the previous majority group could count on suddenly becoming (very) second-class citizens, up to and including persecution, forced exile, or simple massacre.
It's obviously not a horribly good solution. In the present era, we're seeing this come up again in the form of a conflict between (for lack of better words) "Western Society" and "Islamic Fundamentalism;" two groups who used to live far apart, suddenly turned into next-door neighbors by television and travel, with very different ways of dealing with one another, right now fighting to have absolute say over how life should be lived in their own areas.
However, a few hundred years ago - quietly, at first, in 17th-century Europe, and then throughout the Western world - a fundamentally new idea about how to handle this came up. This is the idea of civil law: that there should be a civil administration separate from the religious and cultural laws of the individual tribes.
The idea of civil law, as I understand it, is this: The role of the government is to maintain civil order, and to that end it ought to establish and enforce such laws which assist this, but no more. (So for instance it is in the common interest for the government to prohibit murder and make contracts enforceable.) In all matters where reasonable people can disagree, or where there is otherwise no compelling government interest in legislating, the government ought not to legislate. Individuals are free to enter into covenants and laws more restrictive than that imposed by the government, and the government agrees to not enforce anything beyond its own laws.
For an example of this, consider the laws of marriage and divorce. A marriage ceremony is really two ceremonies: a civil marriage, which is in essence a type of contract between the two partners, and a religious ceremony. The government sees a common type of agreement and formalizes it in its law of contracts; the civil marriage is useful for handling matters of property, insurance and so forth. The religious ceremony is the ceremony for the group in which the people belong; it represents an additional set of restrictions. In a case of divorce, therefore, the civil marriage is simply a contract and can be severed by the mutual consent of the partners; this is the only aspect that the courts can (or should!) deal with. If the partners had an additional religious marriage, there are further issues: as far as the Catholic church is concerned, for example, the religious marriage is indissoluble, and no matter what the legal documents say the church still considers the two people married, whereas in many Protestant denominations, there is a procedure for dissolving a religious marriage as well.
The key point is this: It is in the government interest, for regulating matters of contract and so on, to define a civil contract of marriage. It would not be in anyone's interest if the government were to define this contract to be a specifically Catholic or Protestant contract; at least, not if both Catholics and Protestants are expected to live in the same country together, under a common civil law.
The purpose of the civil law therefore is to provide a uniform backdrop against which multiple groups may live together, essentially bound by the common agreement to preserve the public order. It means that the religious laws of any particular community will not be enforced by the civil courts. (Although if two people belong to the same community, disputes they have in matters of their own communal law can be resolved by courts of their own community, which the parties agree to honor.) Conversely, the practice of any individual community will not be obstructed by the civil courts, so long as it does not threaten the public order.
I believe that this is one of the basic principles underlying the American Constitution, most visibly in the ninth and tenth amendments, but also in the first amendment's establishment clause. And most importantly, this is the idea which has allowed a broad range of social groups to come to America and live together, despite not always agreeing.
The alternative to civil law is something almost too frightening to contemplate. Right now, I can live next door to a devout Shi'ite and a fervent atheist, and we agree (despite having three rather different religious views) not to kill each other, and can even transact business if we wish, or sit in the same coffee shop. If any one of these religious laws were to be enacted into the civil code - well, consider the Taliban and extrapolate it. Yes, there are many groups which would favor different flavors of the above.
