It is an article of faith for both the religious right and religious left that some political situations demand action, and to hang back is to leave Christian duty unfulfilled. Exactly what those circumstances are is contended, and there is wide variety of approach and solution, but the necessity for Christians to act in the larger world to influence the larger human systems is rather taken for granted. It is a very western, especially American, approach.
Evangelicals point to William Wilberforce devoting his life to the elimination of slavery in the British Empire, or Quaker Abolitionists. Liberation theologies would focus rather on confronting the powerful qua powerful on behalf of the poor qua poor, assuming injustice on the basis of their simple existence. The religious left would insist that righteousness be demanded of government in issues of war and care for the poor; the right on issues of biological interference.
I imagine I don't need to describe this exhaustively. Others have outlined these things far better than I could. I recommend First Things if you like to read religious/political discussions among people who are more than talented amateurs. (Most blogs that touch on Christian issues are more exhortatory than thoughtful, providing evidence for familiar POV's. This applies to the essays by clergy as well.) I tend to hang out among the talented amateurs myself.
I also doubt I need to spend much time on the stories of Amos, Jeremiah, and Moses to illustrate Biblical examples of some political involvement, or the NT verses outlining what issues Christians should be concerned about. You know them or could find them pretty quickly. God told the Israelites to organize their society according to just principles, and repeatedly tells individuals to act with justice and mercy toward others.
But is the call to confront larger injustices placed upon all Christians? When there is injustice in one's society, is the Christian obligated to become involved? Assuming the Christian picks the right side of the argument, is involvement always laudable? In the time of Amos, were all who lived in the society obliged to pressure the rulers to make changes, or is that just a retroactive imposition of a value from our western tradition of self-rule?
Dietrich Bonhoeffer long held back from political involvement even in extreme circumstances. I don't doubt that he was permitted to act in Christian conscience, or that Wilberforce, or John-Paul II, or civil-rights workers were permitted their expressions in this sphere. But the idea that if you do not become active you are "participating in the system (leftspeak)," or "tolerating evil (rightspeak)" - perhaps that idea is nonsense. Perhaps such involvements are generally discouraged for most Christians, and only allowed under specific direction of the Holy Spirit.
I ask this because I see how quickly the political goals seem to take over the religious ones in public action, and I figure God knows that tendency in us.