Government will be most successful, Milton Friedman contended, when it acts as an umpire or referee enforcing the formal procedural rules of the game. When it begins to attempt to affect substantive outcomes through active interference, it sets citizens against each other and threatens the social cohesion necessary for the broader society. 
The formal rules government should make and enforce can be found in the fundamental purposes of law. None of us is free if we do not have basic justice and order. Martin Luther wrote in On Secular Authority that men and women need a lawful order in the same way they need food, air, and water.  When we read news accounts about people living in zones of extreme oppression and lawlessness such as have existed in Sudan due to ethnic hatreds or in Mexico because of drug cartels, we realize that the innocent men and women living in those places cannot do much more than survive. They cannot build any kind of a life, because whatever they do can be destroyed or stolen at any time. Surely, the most common denominator of justice is preventing and punishing freedom-destroying evil perpetrated by those who do not recognize even the most basic duties of human beings toward each other. If a government cannot accomplish this goal, then we call it a failed state.
Recognizing this reality, Luther construed the Sermon on the Mount to mean that the Christian must suffer any assault or insult to his person but should always act to protect his neighbor. Government has been ordained in order to restrain predatory, evil men and to prevent them from victimizing everyone else. On that logic, a Christian could certainly serve under the government, and even take men's lives when acting with authority to protect the innocent. 
This reasoning leads to the conclusion that those who do wrong make themselves justly vulnerable to restraint, coercion, and correction by the state. If some men by their unrighteous acts have made themselves fit subjects for coercion and restraint, then what does that say about those who do not commit wrongs against others? The logical corollary is that those who do not commit wrongs should be largely free and uncoerced. They have earned that right because they govern themselves. In other words, if one does justice to others by not harming them through force or fraud, then one should be able to live largely free of government coercion and expect protection from wrongful coercion by others.
The Primacy of Justice
Why do I start with the punishment of evil and reason back to the freedom deserved by those who do not do wrong? The reason is that we more readily identify justice through its violation and remedy than we do through positive visions. We know when we have suffered an injustice that requires a remedy. We are far less certain about whether positive conditions of justice have been met. The common basis of justice is understood in its breach.
Order, justice, and freedom are clearly related. Justice results from enforcing a moral order, which protects the freedom of human beings from malignant interference. We are able to live together in peace and freedom with the government standing by to exercise coercion and restraint upon those who would do wrong.
What about equality, which is also associated with justice? The most realistic kind of equality we can achieve is equality before the law. Every citizen should be able to expect the same treatment by the government. Liberty and protection for him who lives rightly. Coercion and punishment for him who does wrong.
Is there justice enough in providing equality before the law, freedom, and protection from those who would do evil? Many who struggle in ungodly disorder ruled by the strong would jump at the chance to make a life under such conditions. They yearn for their officials to enforce the peace. As these oppressed people hope for justice, they are looking for the government to perform its God-given function in restraining these evil men who willfully commit murder and foment mayhem in local communities. Justice will be done when the government puts down this satanic rebellion against both earthly and heavenly kingdoms.
But others would earnestly reject such a conception of justice (and the corresponding role for government) as too limited. Equality before the law is not enough, they might say, because it results in substantial inequalities in the experience of life. They are right to be concerned but wrong to pin their hopes primarily on government. The special nature of government is found in its legal monopoly on the use of coercive force. Such a unique weapon should be used only when it is clearly justified.
Attractions and Problems of Social Justice
Where I have left things so far will be a source of great frustration to many well-intentioned people. Michael Sandel, professor and teacher of the famous "Justice" course at Harvard, would likely be one of them. He divides political thought into two primary camps. One is based on the abstract, choosing self that guards freedom of decision and action fairly zealously against the notion of group-imposed duties. The other proceeds from the situated self that fully accepts the great solidarity it should feel with other selves in a community and should easily accept non-consensual duties that attach for no greater reason than that one is part of a particular group of people at a certain time. Community is like family in this account.  The situated self, in this account, should feel a Bobby Kennedy-esque drive to use government to redistribute wealth for the good of the community.
Christians who push for collectivist ideals of social justice are, I think, motivated by this account of the situated self who sees himself wedded in solidarity with other members of the community and very much ready to put the government in service of this bond. The partisans of the situated self do not view redistributive taxation and the social control of business as potentially dangerous coercion. Rather, they see virtue at work. For Christians, this view can be very attractive and it has proven so for young evangelicals, especially.
If the situation were as Sandel presents it (basically an either/or between a cold, impersonal freedom and a rich, warm-hearted nicely coercive government), then I would probably feel constrained to opt for the latter choice. But I believe that Sandel commits an error by putting the burden of social solidarity on law and government. What if government is very good at providing the more limited type of peace, order, and justice to which I referred earlier, and is much less good at creating the conditions for some kind of idyllic vision of justice between persons that requires continuous government intervention and readjustment of circumstances? What if other strategies could be placed in the service of civic affection and solidarity?
What Is the Role of Government?
One of the great questions of political philosophy has been whether government should concern itself primarily with small government in the form of something like a mutual defense alliance or if it should instead be far more ambitious about achieving some great dream for all people. The question, it turns out, is a false one. Government is armed with the powers of coercion and force because it must be in order to do the job God has given it, to frustrate the designs of those who would do evil. The broader society does not necessarily require those same weapons in order to achieve its goals. Nor is the use of those weapons well-justified in many instances. We should be far more keen to work in the voluntary sector than in the coercive one.
This is not merely a call to a more ingenious strategy of non-profit organization. Our non-governmental options start at a much more basic level. We occupy many special offices in this life. The offices are things such as son, daughter, brother, sister, husband, wife, father, mother, grandparent, uncle, aunt, cousin, neighbor. How many of our problems could be addressed by simple faithfulness to the tasks presented to us by the nature of these offices? And how many of them could be addressed by the church in better ways than calling for enormous government programs, thus inviting a hammer into places where warm hearts and hands would better serve?
Let the force of law serve where it works best and let the rest of us accept the heavy, but ultimately joyful, burdens that attach to life as a responsible human being. And let the church preach in such a way that we recognize and act upon our responsibilities rather than relying on the magic of some bureaucratic plan to relieve us of them.
 Milton Friedman, Capitalism and Freedom: 40th Anniversary Edition, (Chicago: University of Chicago Press, 2002), 23-24.
 Martin Luther, "On Secular Authority," in Luther and Calvin on Secular Authority, ed. and trans. Harro Hopl (Cambridge: Cambridge University Press, 1991), 13-14.
 Ibid, 15.
 Michael J. Sandel, Justice: What's the Right Thing to Do?, (New York: Farrar, Strauss, and Giroux, 2010), 208-243.