(The following rant was written rather quickly as I was pressed for time. There are probably errors and also certain ideas that will need further clarification. This is merely my view on the issue of self-defense based on My Life So Far, and like most issues that require considerable thought, is subject to change.)
Like Ash, I am an evangelical Christian holding to a reformed, covenant theology. I believe that covenant theology is the proper hermeneutic through which to interpret the Bible. For this reason, I think it is particularly important to emphasize the unity of the Old and New Testaments, even when talking about an issue like self-defense.
It is often pointed out by pacifists that based upon the example of Jesus, there can be no advocation of or participation in violence on the part of the Christian. While this argument is usually well-intended, I do not believe it is grounded in a good understanding of doctrine.
There are numerous examples within the Old Testament of God's people defending themselves with weapons. There are also countless commands in Leviticus and other books concerning the dispensation of justice. God was explicit that those who took life forfeited their own. He was not talking about executors of justice; He was talking about murderers. Murderers were to be killed, and to do so was not itself an act of murder, but rather a cleansing act. It atoned for the innocent blood that had already been spilled. We've often heard "an eye for an eye, a tooth for a tooth" as being foundational to Old Testament Law. God also taught the law of "blood for blood."
Yet when we get to the New Testament, we see Jesus advocating love, mercy, deference, personal sacrifice, even the laying down of our lives. This new teaching can almost seem at odds with the rigid system of justice taught in the Old Testament. To "fix" this problem, Christians often make the mistake of saying that the Old Testament Law was abolished with the coming of Jesus, but this is not true. In the Sermon on the Mount, Jesus specifically states that he did not come to abolish the law, but to fulfill the law. In other words, God's sense of justice did not change between Old Testament and New. He is still the same God, and His son Jesus became the only human to ever live up the Law's demand of absolute righteousness, which none of us could ever attain.
What many pacifists get hung up on is the fact that Jesus never exercised self-defense. Judas betrayed Him, the Pharisees mocked and tortured Him, Pilate had Him beaten and crucified, and yet at no point did Jesus demand that he be treated justly. Therefore, if we are to follow Jesus' example, we are never to stand up for ourselves, particularly if it requires violence, right?
Well, no, actually. Where many Christian pacifists go wrong is that they seem to forget that Jesus' mission was slightly different from the one he gave his followers in the Great Commission. The whole purpose of Jesus' incarnation and life was so that he could die, and thereby atone humanity's sin. It would not have made any sense for Jesus to take up the sword against his persecutors, because his mission on this planet required for him to die. Jesus was focused on appeasing the requirement of God's eternal justice and allaying His wrath, and this mission superceded any concern for Himself. As a Christian, my calling is not to physically die for mankind's sin, though it will often involve dying to myself. And while I can't rule out the possibility that I may someday have to physically die for my faith, I don't believe that refusing to defend myself from a violent mugger based on moral principle is the same thing as "dying for your faith."
As an example, imagine that the United States became a fascist or communist state that outlawed Christianity, and the consequence of faith was death. In its "mercy," however, the government gave each Christian the opportunity to recant the Lordship of Christ and go on their merry way. Imagine that a Christian who is given the opportunity to recant refuses to do so, affirming their status as one of the elect. And imagine that they are then brutally tortured and killed, but at no point do they deny their God. In this case, I do not believe that self-defense would be justified, even if the Christian in this example was able to do so, because there is no greater witness for the cause of the Gospel than to lay down your temporal life in affirmation of an eternal life. This kind of witness was one of the reasons that the early Church spread over the ancient world like a wildfire driven by divine wind. In the ancient arenas, pagans saw Christians who believed in a truth that was worth dying for. This was a radical and mysterious thing to people who for the most part had probably not found anything worth living for, much less something that was more important than their own life. The people who mocked Christians as they were torn apart by Lions were soon joining the ranks of God's people. The arenas of the Roman Empire possibly did as much to advance the Kingdom as all the teachers and preachers of the early Church.
As another example, we have a Christian who stops at the ATM to get some cash late in the evening. As he is walking back to his parked car, he is surrounded by members of a vicious local gang. Our Christian is an alert fellow, and he senses the predators before he sees them. Out of the corner of his eye, he sees one of the gang members reaching into the front of his pants for a gun. The Christian has his own firearm concealed on his person, and he has trained for exactly this type of situation (and has already chided himself for getting into it). Our protagonist knows that if he doesn't defend himself, he will almost certainly die. He also knows that if he does defend himself, some gang members will almost certainly die. While the Christian can't help but feel that it would be just to defend himself from a group of people intent on his murder, he also knows that "thou shalt not kill," and that Jesus was meek and sacrificial in everything he did. Could God forgive him for taking someone's life to preserve his own? In this moment of indecision, the Christian is gunned down. His money, gun, and car are stolen and he is left to die.
I believe that Christians are first and foremost called to be Christ's ambassadors, so as an ambassador, the first question I have to ask is: How did the Christian in the second example advance the cause of the Kingdom? Did the gang members come to a better understanding of God and the Christian life by this "meek" Christian's death? Are they more likely to become members of the Kingdom because our "sacrificial" Christian didn't defend himself? I really can't see how, mainly because the Christian wasn't murdered for his faith. In fact, the gang members probably had no idea their victim was a Christian. He was just a guy with money and a car. In one sense, the murder was "nothing personal," and because the gang members didn't personally know anything about their victim, his death does nothing to advance the cause of the Kingdom. In fact, this is one case where I believe the Christian had an opportunity to stand up in the face of evil but failed. For better or worse, he was in a situation to which he could have justly responded. Death would have been a proper consequence to the actions of gang members who are bent on terrorizing the innocent. While the loss of life is always unfortunate, I do not believe the taking of it would have been sin in this example. The protagonist was a citizen of both a temporal society and an eternal Kingdom that could not survive without the advocation and practice of justice. Unfortunately, our Christian didn't understand that mercy does not replace justice, but complements it. The result is that rather than advancing God's Kingdom, the Christian's loss of life only empowered some gang members with a car, an extra gun, and some money. Off they go to find their next victim, one who is probably less capable of defending themselves.
When it comes to the issue of self-defense, I believe that the most important question the Christian can ask themself is whether their action will advance or hinder God's Kingdom. If someone is intent on taking my life because of the faith that I espouse, then I will have to consider giving it to them, because my primary concern is for their soul and the witness of the Gospel. And how can their unbelief be anything but shaken in the presence of a belief so deeply held that we are willing to die for it?
But laying down my life to the stranger who terrorizes for the sake of material goods or lusts is not redemptive. Life—particularly life that is bent toward goodness—is worth more than anything material, and only the soul is worth more than life. If by laying down your life you have the opportunity to win souls by bearing witness to the Gospel, then by all means do it. For anything less, allow justice and conscience to be your guide.
Psalm 106:3
Blessed are they who maintain justice, who constantly do what is right.
Proverbs 21:15
When justice is done, it brings joy to the righteous but terror to evildoers.
Isaiah 1:21
See how the faithful city has become a harlot! She once was full of justice; righteousness used to dwell in her—but now murderers!
Matthew 23:23
"Woe to you, teachers of the law and Pharisees, you hypocrites! You give a tenth of your spices—mint, dill and cummin. But you have neglected the more important matters of the law—justice, mercy and faithfulness. You should have practiced the latter, without neglecting the former."
Romans 13:3-4
For rulers hold no terror for those who do right, but for those who do wrong. Do you want to be free from fear of the one in authority? Then do what is right and he will commend you. For he is God's servant to do you good. But if you do wrong, be afraid, for he does not bear the sword for nothing. He is God's servant, an agent of wrath to bring punishment on the wrongdoer.
2 Corinthians 5:20
We are therefore Christ's ambassadors, as though God were making his appeal through us. We implore you on Christ's behalf: Be reconciled to God.