So, here is the first thing that I think Christians should think about when they consider politics:
God is King
A bit of a no-brainer, perhaps, but it has some important implications. The particular image (King) is not so important as the simple fact that it conveys: that God is in charge, that he rules. Let me expand that in a few directions, and look at some of the implications for the political sphere.
a) God is King in every sphere of life
Which obviously means that what happens in the political sphere is not something outside of his interest. Politics is not, and cannot be, a neutral sphere with regard to God. That is the problem with the "secular state" theory. Now, at one level, as we'll see in a later post, I approve of the idea of a "secular" state, if that simply means that the state ought not to try to force a particular religion (even the true one!) on people. However, the political realm is inhabited by people, and people are never neutral when it comes to God. So, a Christian politician cannot switch off their Christian convictions when it comes to political action; neither can a Christian voter (or potential voter) ignore the teaching of the Bible when it comes to their involvement in politics.
b) Only God is King
Because God does not (ultimately) delegate his authority, no government can claim absolute authority over the governed. The state cannot claim God's place - if it does, it is justly resisted (although what form this resistance might take is another question). Note in passing that this also counts against any form of Papacy. God rules in the person of his Son Christ (see Psalm 2), and he does not give all his authority to any human agency.
c) God is King over every individual
This point cuts two ways. Because God is King over every individual, every individual is ultimately answerable to God and not the state or anyone else. On the other hand, because God is King over every individual, there is no individual who can claim moral autonomy.
I think that the simple truth of God's active rule of the world, including the sphere of human government and politics, immediately limits that sphere. It is not ultimate, and it cannot claim ultimate control or allegiance. Already, I think we're pointed in the direction of a very limited state. How limited remains to be seen.