This is explained in The Ethics of Martin Luther (Paul Althaus; pages 75-76) thus:
Luther replaces the distinction between matters that affect our own person and matters that affect our office with a distinction between matters that concern the Second Table of the Law and matters that concern the First Table. This is a distinction between matters which affect us as citizens of this world and matters which affect our faith and confession. . . . This means that whenever a Christian is threatened and attacked as a Christian (that is, for the sake of the Gospel and thus for the sake of Christ), he does not defend himself; rather, he is ready to suffer injustice and violence without resisting and to abandon joyfully all that he has -- even his body and his life. But in secular matters, when his suffering is not for the sake of the Gospel, he may turn to the authorities for help and demand justice and protection. If his request is not granted, then he must suffer in the secular matter too.
This corresponds to Luther's opinion of self-defense. If the authorities persecute the Christian because of his faith (that is, in matters related to the First Table of the Law), he does not resist but instead suffers everything, including death. If a thief or robbers uses violence against him, however, the Christian as a "citizen of this world" ought to defend himself.
Luther thus establishes my right to defend myself when my life is under attack by asserting that it is no longer a private action for my own personal benefit but an official action of the authorities which I perform in an extraordinary way, that is, as a substitute for the official authority.
You see, Martin Luther splits the human's being into two parts, one secular and one sacred. I don't see how this fits with scripture. For example visit Colossians 3:17, "And whatever you do, whether in word or deed, do it all in the name of the Lord Jesus, giving thanks to God the Father through him," and to a lesser extent 4:5, "Be wise in the way you act toward outsiders; make the most of every opportunity." Also see 1 Corinthians 10:31, "So whether you eat or drink or whatever you do, do it all for the glory of God."
We are also told in Romans 6 and 8 that we are dead to our old nature and given a new nature. If the old, secular nature is dead there is no secular reaction coming from the Christian, only the sacred action coming from the live that is lived in the Spirit of God.
The point here is that we are always to act as God's representative, so if we are attacked, it is not a pagan who is responding, it is a Christian. When Jesus said to "turn the other cheek" (Matthew 5:39) he is addressing the original law given ("eye for an eye") which is a "Second Table" act. It is an act committed to a person not in the role of Christian but in the role of human. And Jesus never makes a distinction that this "evil person" (same verse) is assaulting you because of your faith.
Jesus says to "love your enemy" (Matthew 5:43-48) because even the pagan loves those who love him, so as a disciple of the Risen One we are to also love our enemies. He concludes by saying, "Be perfect, therefore, as your heavenly Father is perfect." For God loves his enemies: Romans 5:10, "For if, when we were God's enemies, we were reconciled to him through the death of his Son, how much more, having been reconciled, shall we be saved through his life!" Colossians 1:21, "Once you were alienated from God and were enemies in your minds because of your evil behavior."
In conclusion, Martin Luther and other theologians are correct in their reading of Scriptures as it calls us to love our enemies and to turn the other cheek, but I question their designation of this to only martyrdom.
I will close with a passage from 1 Peter 2:21-23 where we are told Christ is an example and we should follow in his steps. It is the example Christ gave us that is difficult to accept:
To this you were called, because Christ suffered for you, leaving you an example, that you should follow in his steps.
He committed no sin, and no deceit was found in his mouth. When they hurled their insults at him, he did not retaliate; when he suffered, he made no threats. Instead, he entrusted himself to him who judges justly.