It has never been clear to me where the line is, because, again, the New Testament doesn’t answer it. From the perspective of the NT, the individual is supposed to devote his life to the exercise of his gift of ministry and from such seeking of the kingdom his personal answers emerge.
The post-apostolic church, though, struggled with this question a great deal and a strong tradition developed that Christians shouldn’t be engaged in directly coercive occupations: i.e. soldiers, political offices, etc.
That tradition submerged with Constantine, of course. (And that sure worked out well.)
Later, such groups as the Anabaptists and their descendants decided they couldn’t be coercive. Their traditions continue, and i have sympathy for their stands.
I personally could not hold political office.
I vote. I vote for minimal government and lower taxes. I do not vote for candidates who want to pass laws which will impose Christian values on non-christians, except where those values will protect others from having values imposed by force on them (abortion, for example).
So I am almost a libertarian, I suppose you could say, and almost a pacifist. But not quite, in both cases: I don’t share the view held by many libertarians that taxes are illegitimate, since both Jesus and Paul had no issue with taxes. And you obviously can’t have a government without coercion — in fact, the government simply IS the sword.
I don’t mean to give you a tour of my mind, I’m just trying to answer as honestly as I can.
One more thing: you asked “what sort of decisions should Chrsitian judges hand down?”
Answer: those that are in accord with the laws as they exist. if those laws violate his Christian conscience, tough. He has no right (and certainly no biblical commission) to impose his Christian values on others if those values are not reflected in the laws. If he can’t rule by the written law, he should step down.
A legislator is different, because his role is different. He has no duty to vote for “Christian laws” — there is no such thing — and his role is awkward because he has no model to go by. He is left with a few terse statements from the NT (Paul, mostly). At the same time, many moral laws are so clear they need no further analysis (thou shalt not kill) and any law which violates that sort of clear line he cannot support.
I don’t believe a legislator could follow the NT carefully and succeed. He would be voted out of office forthwith in almost any jurisdiction in he land.