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A man armed with a knife was up a tree making a disturbance outside a home... Deputies headed to the home after receiving a 911 call … On their way to the scene a second alert came in ... The man was trying to break in by throwing a brick through a locked glass door that led to an enclosed porch ...
After getting to the porch, the man beat on the glass door to the home screaming to be let in ... The homeowner shouted that he was armed ... The man picked up a dumbbell on the porch, threw it through the glass door, and entered the home ... The homeowner fired his .45 semiautomatic, shooting the suspect once in the arm. 
This was not a turn-the-other-cheek moment.
The issue of self-defense is important. Human nature has not changed, and neither has biblical law regarding how we as Christians respond to it at the personal level or in terms of threats of bodily harm.
Have you noticed that Jesus does not say, “whoever slaps you on one cheek, turn the other to him also” (Matt. 5:39)? Here’s the passage. Notice the difference:
[D]o not resist an evil person; but whoever slaps you on your right cheek, turn the other to him also.
Why the right cheek? Peter Leithart comments:
[Jesus] is not describing a situation in which our lives are imperiled. He is not talking about an adversary coming at you with deadly force. The law provides for self-defense and defense of your house, family, and property if someone is breaking in. Jesus is not eliminating that. He’s talking about honor and dishonor, insult and shame. If I receive a slap on the right cheek, either the slapper has slapped me with his left hand or he is slapping me with his backhand. Either way, it’s insulting. In Israel, the left hand is reserved for dirty work—using the bathroom and such. So, getting slapped with the left hand is insulting. But a slap on the right cheek with the right hand is a backhanded slap, an insulting slap rather than a danger to life and limb. The person who slaps you with the back of his hand is treating you as a slave, as an underling. He is not treating you as an equal. He’s sweeping you away like a flea. 
Jesus is telling His disciples and us to walk away from something that might escalate into a feud and beyond. “Jesus is forbidding us to enter into the cycle of vengeance and counter-strike….” 
There’s a big difference between slapping someone across the face and someone wanting to take a baseball bat to your head or the head of your wife and children. Self-defense is a biblical option in such cases. Consider this passage from biblical case law:
If the thief is caught while breaking in and is struck so that he dies, there will be no bloodguiltiness on his account. But if the sun has risen on him, there will be bloodguiltiness on his account. He shall surely make restitution; if he owns nothing, then he shall be sold for his theft (Ex. 22:2-3).
The homeowner can assume that someone breaking into his house at night has nothing but bad intentions. He may be armed or not. The homeowner does not have to ask any questions to find out. The homeowner can respond by striking the intruder “so that he dies.” If this happens, even if the attempt was only theft (unknown to the homeowner), the homeowner is cleared of all guilt in the thief’s death.
Your understanding of what it means to be "under authority," will be shifted and your view of ruling and rulers will be enriched. You will come to see the inescapable nature of government, and how man tends to impose authority from the top-down while God's governmental structure is bottom-up.
It’s possible that Jesus had the Old Testament case law in mind when offered this injunction to His disciples: “But be sure of this, if the head of the house had known at what time of the night the thief was coming, he would have been on the alert and would not have allowed his house to be broken into” (Matt 24:43).
Daytime is a different story because potential victims can make a better assessment of intent. If a man enters a building with a shotgun, as happened in a Texas church, killing the armed man before he kills anyone is the right thing to do. Being loving, peaceful, just and generous, and self-giving do not apply.
The story of David and Goliath is helpful since “five smooth stones” and a “sling” are the closest equivalent to a handgun we can find in the Bible. David always seems to have been armed with his sling in case of unforeseen danger to himself. There was no way he could run home to get it when a lion or a bear was about to attack his flock (1 Sam. 17:31–37, 41–54).
But of course, you rarely know when someone is going to break into your house or decides to kill people in your church, therefore, you must be on guard all the time.
In another passage, Jesus is teaching by analogy:
When a strong man, fully armed, guards his own homestead, his possessions are undisturbed. But when someone stronger than he attacks him and overpowers him, he takes away from him all his armor on which he had relied and distributes his plunder (Luke 11:21).
A fully armed strong man is a deterrent to a thief. It’s the fact that the strong man is armed that protects the potential thief from being harmed. Another strong man will think twice about ever trying to rob or harm someone who is armed.
The most famous New Testament passage is a command of Jesus for His disciples to sell their garments and buy a sword (Luke 22:36–38). It shows that being armed was a norm for that time, and Jesus does not object.
Peter impetuously uses his sword against a servant of the high priest (John 18:10; Matt. 26:51; Luke 22:50) who had come out with a crowd armed with clubs and swords (Luke 22:52). Under normal circumstances, swords were permissible for self-defense, otherwise why did the “chief priests and officers of the temple and elders” have them?
The disciples lived in dangerous times (Luke 10:29–37). Furthermore, the Romans didn’t seem to have a problem with their subjects (the Jews) owning swords.
Gun-Free Zones are soft targets for people who have no regard for the law. The gunman who killed the people in Luby’s Cafeteria in 1991 had broken the law by bringing a firearm into a place where the law said it was unlawful. Murderers are, by definition, lawbreakers.
Now to the question. Should churches, for example, ensure that there are armed and trained people at every service? Absolutely! Christians might say, “But we should put our trust in God.” God has given us the ability to reason and assess the times like the sons of Issachar, “men who understood the times, with knowledge of what Israel should do, their chiefs were two hundred; and all their kinsmen were at their command” (1 Chron. 12:32).
Consider the following from the book of Nehemiah:
But when Sanballat, Tobiah, the Arabs, the Ammonites and the people of Ashdod heard that the repairs to Jerusalem’s walls had gone ahead and that the gaps were being closed, they were very angry. They all plotted together to come and fight against Jerusalem and stir up trouble against it. But we prayed to our God and posted a guard day and night to meet this threat.
Meanwhile, the people in Judah said, “The strength of the laborers is giving out, and there is so much rubble that we cannot rebuild the wall.”
Also our enemies said, “Before they know it or see us, we will be right there among them and will kill them and put an end to the work.”
Then the Jews who lived near them came and told us ten times over, “Wherever you turn, they will attack us.”
Therefore, I stationed some of the people behind the lowest points of the wall at the exposed places, posting them by families, with their swords, spears and bows. After I looked things over, I stood up and said to the nobles, the officials and the rest of the people, “Don’t be afraid of them. Remember the Lord, who is great and awesome, and fight for your families, your sons and your daughters, your wives and your homes.”
When our enemies heard that we were aware of their plot and that God had frustrated it, we all returned to the wall, each to our own work.
From that day on, half of my men did the work, while the other half were equipped with spears, shields, bows and armor. The officers posted themselves behind all the people of Judah who were building the wall. Those who carried materials did their work with one hand and held a weapon in the other, and each of the builders wore his sword at his side as he worked. But the man who sounded the trumpet stayed with me.
Then I said to the nobles, the officials and the rest of the people, “The work is extensive and spread out, and we are widely separated from each other along the wall. Wherever you hear the sound of the trumpet, join us there. Our God will fight for us!” (vv. 7–13).
While they trusted God and prayed, they also understood that they were responsible for their immediate welfare by posting a guard (v. 9). Notice that while Nehemiah said, “Our God will fight for us,” we’re also told that “half [the men] were equipped with spears, shields, bows, and armor.” This is not a contraction. Prayer is not enough unless it’s the only act that we have at our disposal.
They never let down their guard.
So, we carried on the work with half of them holding spears from dawn until the stars appeared. At that time, I also said to the people, “Let each man with his servant spend the night within Jerusalem so that they may be a guard for us by night and a laborer by day.” So, neither I, my brothers, my servants, nor the men of the guard who followed me, none of us removed our clothes, each took his weapon even to the water (vv. 21–23).
One more thing, when Israel’s enemies heard that the men were armed and on guard, they had second thoughts about attacking. Human nature has not changed since Cain killed Abel. What has changed in our culture is a disregard for human life.