It's true that Christ's earliest followers were not exactly well-known in the halls of power. But why do you think that Christ's teaching regarding a government operated by Christians would have differed from His actual teaching? Why do you think He would have said something other than "render unto Caesar" if those who controlled the government were counted among His followers?
Jesus was in general an advocate of a pretty transformative philosophy. One can't really be a follower of Jesus without being a follower of Jesus in all things. Luke 14 is good here. A government run by Christians is going to be a Christian government - it will be set up to further Christian goals. That's not to say that it's going to require that all citizens be Christian, but it's going to be very concerned with all of the things that Christians are supposed to be very concerned with, such as the welfare of the worst off, among other things. It will not do things that Christians would prefer it not to do, simply because Christians have the power to choose what the government is going to do.
This renders the whole render unto Caesar thing superfluous. There can be no prima facie conflict between duty to the state and duty to God. If Christians in government can in good conscience levy taxes, then Christians can in good conscience pay those taxes. Christians in government can't levy an un-Christian tax. They can't pass un-Christian laws. And so on. Government is just a way for people to act collectively, and if Christians control it then it's just another Christian institution. That's not to say that the state
is the church, but the state is, if the Christians understand Christianity correctly, the state Jesus would want. Maybe Jesus is a federalist who believes in the second amendment - it's a little hard to say since he never really articulated a philosophy of exercising earthly power - but whatever a Christian government is doing, Christians can wholeheartedly support it (I'm very much not saying that a government deserves the support of Christians just because it claims to be Christian).
I definitely agree with you to the extent that such arguments are used to shirk one's moral responsibility. To the extent that we're talking about public policy, though, I'm not sure how Christ would have responded to such arguments. As I said earlier today, He never specified whether His teachings should drive public policy.
I think it depends what you mean here. Yeah, I don't know what Jesus would have said to a poor person who suggested trickle-down economics as government policy. But I have a really hard time imagining Jesus being okay with a wealthy person advocating trickle-down economics. It sounds self-serving at best and stinks of blackmail at worst. I mean, any person who is
still wealthy is doing Christianity wrong already. Presumably they're not to be trusted when they say that the best Christian government is one that works directly to keep them wealthy. That's not really an argument that can be made in good faith.
I don't think that's the only way to square inequality with Christianity. For instance, it could be that a Christian believes involuntary takings are immoral. It could be that a Christian does believe that inequality is immoral, but doesn't wish to impose his or her moral views on the subject through government action.
This is fair enough. I think there's still a long-term problem here in that Christians can wield a whole lot of uncontroversially legitimate nongovernmental power, the effects of which are hard to distinguish from government action. Imagine something like the reverse of the Reconstruction-era South. Christians wield so much economic power that they ought to be able to pretty easily ensure that they're not giving lots of profit to people who don't make good use of it. But we don't actually see this ever. People who run big companies consistently get really rich. But this is phenomenally un-Christian - we agree that Jesus would not be happy with the modern rich, even if for all we know he would have thought it wrong to take their stuff.
But so, why not work to make sure that the CEOs and shareholders of big companies
are Christians? Instead of supporting KFC and Hobby Lobby (chosen because they're perversely held up as Christian companies in some way, and because they show that self-identifying Christians are at least
willing to throw their economic weight around to cause change), why not support companies with Christian investors who, after all the charity they give in accordance with Jesus' teaching, live modest lives on the level of the typical citizen? It seems hard to explain why this doesn't happen, except that maybe there are just very few real followers of Jesus out there.