How exactly is the Christian to view the state? The answer to that question answers a slew of other ones, including, perhaps, the thorny question of gay marriage sanctioned by the state.
Three options jump out at me as I think about Christianity and the state:
1. The medieval Catholic view
2. Calvin's view
3. Luther's view
The medieval Catholic view has the state below the church. If we were to draw an org chart, the state would be at the bottom, the church in the middle, and God at the top. In this scheme of things, it clearly makes sense to speak of a Christian state.
Calvin's view is a little different. The church and the state are not in a hierarchical relationship. Each answers to God separately, but the implications are not what you might think. Because God invests government with authority, governors should be primarily concerned with things like right worship and doctrine. Heresy would absolutely be a punishable offense.
What is similar about the two views described above is that the state is a sacred entity and it is going to be involved in matters of religion.
Luther represents a definitive break. His state is not sacred or confessional. Instead, it is purely instrumental, which is to say that the state has no eternal destiny but it has a job to do. The job is simple: restrain evil. Because of the fallen state of man, sin is everywhere on the earth and without the restraint of rulers the world would be a desert as the wolves preyed endlessly on the sheep. Luther's state shouldn't worry so much about correct doctrine and punishing heretics. That is for the church to handle through persuasion and excommunication. Instead, the state should wield the sword against those who will do evil in the form of violence, theft, and fraud.
Of the three options, it may be clear that I think Luther's view is the best and is of the closest accord with the New Testament. Christ really didn't bring a doctrine of the sacred state, at least not as far as I can tell. The church's mission is far more important than the state's, but we often act as if we believe the state is where all the action is. I think that is a legacy of Calvin. I should also add that I see foreshadows of Locke in Luther, but Luther rarely gets the credit.
Gay marriage comes into the picture because it is of such great moment for Christians involved in American politics. I still recall talking to John Whitehead of the Rutherford Institute in 1998 with a group of fellow summer associates. Whitehead told us gay marriage was already lost. We protested. I think he was right and we were wrong. The question is how upset we should be about that.
If you take Calvin's view of the state, then I think gay marriage is completely unacceptable. The honor of God is implicated in the Calvinistic state and something so clearly at odds with Christian doctrine would be an ultimate affront. The result is that you have to fight and fight hard because the state is a covenant entity and God will punish a faithless people.
If you take Luther's view, the picture is a bit less bleak. Gay marriage is really outside of what the state should be doing, but the honor of God is not on the line because we are only talking about an instrumental entity for earthly convenience. It is quite possible that gay marriage will represent a milestone that immediately fades into insignificance as we discover the whole thing was primarily about making a point rather than about the desire to build nuclear families.
Just to clarify a bit through comparison we can see that in Calvin's world gay marriage would be every bit the problem abortion is. Gay marriage might be even worse than abortion because it runs directly counter to scripture. In Luther's world, abortion would be far more grave because the state is licensing real harm and violence against innocent parties. Gay marriage represents something less troublesome by several degrees.
Love to hear discussion and feedback on this one. The thinking here is early and tentative.
Update: I mentioned three models of the state from a Christian point of view, but there are others. For example, one could embrace a radical reformation view in which the church withdraws almost completely from the state, viewing it as a source of corruption and malignant worldliness.