There are two poles, I think, to Christian attitudes towards the state.
At the one pole is a catacomb mentality. Here Christians think of the state as an unrelievedly secular force, and they seek to render what they must unto Caesar while keeping themselves aloof from its corrupting influence.
At the other pole is what might be termed the New Jerusalem mentality, in which the Christian sees the state as a means to achieve a more just and loving society which will reinforce virtue.
Both of these approaches are, I think, flawed in their different ways. I think that perhaps the New Jerusalem mentality is more dangerous than the catacomb one, but either represents an unhealthy extreme.
The danger of the catacomb mentality is that it fails to seek to bring secular laws and institutions in line with what we actually believe to be just. It’s all very well to say that we know that state to be a secular and potentially corrupting influence and to hold ourselves separate from it, but the fact is that in any society the civil laws and customs will be looked to by many people as a normative form for what is good and just. If we believe that these civil laws and institutions do not in fact conform to justice — that they are in fact unjust — and yet we fail to make any move to improve them, we fail to provide guidance and justice to those around us in society. My holding ourselves separate from society in this way, we do society a disservice, and we fail to serve the truth by telling other of what we believe to be true.
The dangers of the New Jerusalem mentality, however, are even greater. At first we might see seeking to achieve a truly just society via the state as an unmitigated good. And yet the danger here is in allowing ourselves to imagine that a state is a vehicle capable of achieving a truly just and virtuous society — and come to that, imagining that a society as a whole can be truly just and virtuous.
In allowing ourselves to imagine that society as a whole is ultimately perfectable through the means of the state, we both deny human nature (which is itself fallen) and we dangerously tie our ideas about ultimate goodness and justice to laws and institutions which must necessarily be imperfect.
The fact is, no state or social structure can make people behave justly or virtuously, only the individual human will can choose to do what is good. The state can, at best, have laws which prevent people from inflicting the greatest injustices upon one another — laws which must, due to our imperfect human natures, always be backed up by the threat of force. But the real pursuit of a just and virtuous life must be left to individuals, to families and to the other smallest institutions in which we as human being naturally feel real ties to each other as persons. In this sense, a correct understanding of limited government — leaving the most important and personal elements of society and culture to those local and subsidiary organs which are best suited to them — is essential for a right understanding of the relationship between Christians and the state.