But I tell you, do not resist an evil person. If someone strikes you on the right cheek, turn to him the other also. And if someone wants to sue you and take your tunic, let him have your cloak as well. (Matthew 5:39-40)Getting involved in secular matters isn't a sin for Christians. Believers are simply carrying out the responsibilities that all citizens have--whether Christian or non-Christian. Yet believers have to consciously avoid sin and do what Christ expects of them. In contrast, the people of the world don't do what Christ requires.
That's why when Christians fight in a war, file a lawsuit, or impose a punishment, they are functioning in their role as soldier, lawyer, or judge. But within these roles, Christians will want to keep their consciences clear and their motives pure. They don't want to hurt anyone. So they live simultaneously as Christians and as secular people. They live as Christians in all situations, enduring hardships in this world. They live as secular people obeying all national laws, community regulations, and domestic rules.
In summary, Christians don't live for visible things in this life. These things fall under the authority of secular government, which Christ doesn't intend to abolish. Outwardly and physically, Christ doesn't want us to evade governmental authority or expect us to abandon our civic duties. Instead he wants us to submit to and make use of the organizational and regulatory powers of the government which keep society intact. But inwardly and spiritually, we live under Christ's authority. His kingdom isn't concerned with governmental authority and doesn't interfere with it but is willing to accept it. So as Christians and as individuals, we shouldn't resist an evil person. On the other hand, as citizens with responsibilities in society, we should oppose evil to the full extent of our authority.
[UPDATE: In the comments, Bob asked what might be meant by "secular." I tried to answer that. Here's what I wrote (fixed a bit, since I initially wrote it on the fly):
Thank you for your kind comments. I am feeling better and in fact, in a short while, intend to do a little dusting here in the house while my wife handles lawn-mowing duties.
As Luther uses the term, secular, in this citation, I believe that he's referring to offices and functions other than Christian "offices," whether those filled by clergy or laity.
What underlays these remarks is Luther's understanding that God operates in two kingdoms here on earth: the kingdom of the right and the kingdom of the left. The kingdom of the right is composed of those persons who voluntarily and by faith, live under the reign of Christ. They remain sinners as well as saints, but they seek by daily repentance and renewal to live out the new life given to them by Jesus Christ. Their intention to live in accordance with God's will is something they offer up freely out of gratitude for Christ and is not coerced.
However, because we still live in a fallen world, God has also established the kingdom of the left. This is composed of the governments, laws, and regulations, whose coercion is necessary to keep unrepentant humanity in check. Christians voluntarily live under such secular authority out of love for their neighbor. [Luther also says that if there were no kingdom of the left, we Christians would live in a world like lambs among ravenous wolves.]
Accordingly, Luther argues that Christians may, with clear consciences, provide services to governments. He would argue, I'm sure, as the Augsburg Confession states emphatically, that the Church and those holding Church offices, in their capacities as officers of the Church should refrain from taking secular authority, lest voluntary acquiescence to Christ's authority become coercion; lest too, those pastors, priests, or bishops speak and act on their own authority rather than that springing from the Church's true power: the Word and the Sacraments.
There is good reason to be critical of some of what Luther reasons from Scripture. There were too many Lutheran clergy--Dietrich Bonhoeffer is a notable and blessed exception--who felt that acceptance of secular authority meant they needed to go along with Nazism. There were also Lutheran apologists for slavery in the antebellum US.
But this may not be an inherent flaw in Luther's two kingdoms thinking. There must be exceptions when Christians break with secular authority.
A Mennonite theologian, John Howard Yoder, provides a useful gauge by which we can judge whether Christians are free to break with secular authorities. Yoder puts two passages of Romans in tension. First is Romans 13: 1:
"Let every person be subject to the governing authorities; for there is no authority except from God, and those authorities that exist have been instituted by God."
The other is Romans 12:2:
"Do not be conformed to this world, but be transformed by the renewing of your minds, so that you may discern what is the will of God—what is good and acceptable and perfect."
I may have my personal opinions about the actions of secular authorities. As a Christian individual, I am free to vote as a please, write to members of government expressing my views, etc.
But I dare not invoke the name of Christ, nor may my Church, speak out on political issues unless a secular authority asks me or other Christians to live in ways that conform to the fallenness of the world rather than the will of Christ.
Keep in mind that Paul wrote these words during the Roman Empire. He did not, in his office as an apostle of the Gospel, speak out on political issues. The Church of today should show similar reticence, I think. We have bigger fish to fry--making disciples.
If the Church will be about the business of making disciples, those living under the kingdom of the right--Christ's kingdom--will change the world without political power plays, social statements, wars, or coercion. The power of the Holy Spirit to woo people to faith and true righteousness is infinitely greater than the secular world's capacity to coerce what Luther called a "civil righteousness" from resentful citizens. The Church is crazy to go for secular power or to cozy up to politicians when it already has more power at its disposal--through the Word and the Sacraments and prayer--than any government or empire in the history of the world.
That's my windy response, Bob. I hope that it helps.]