What does the Bible say about a Christian carrying a weapon for self-defense?
The Bible teaches in various places that a believer may act in self-defense or defense of others without sinning, including acts of defense that involve taking the life of another when necessary.
Some turn to the sixth commandment's prohibition against murder to claim that the Bible prohibits taking any life under any circumstances, but this is not a correct interpretation. The word murder means, by definition, the unlawful taking of life. Exodus 20 uses the Hebrew word for murder (ratzach) rather than the word for kill (harag) to acknowledge some forms of killing are justified and therefore not sin.
According to scripture, lawful forms of killing include during wartime against enemies of peace (e.g., Numbers 21:1-3), by the government as a just punishment for crime (e.g., Romans 13:3-4), in self-defense (e.g., Exodus 22:2-3), and in defense of others (e.g., Exodus 2:11-12 ).
Even Jesus implied the need for his disciples to prepare for self-defense against those who might threaten them as fellow "transgressors" once Jesus had been condemned, so He gave these instructions:
Luke 22:35 And He said to them, “When I sent you out without money belt and bag and sandals, you did not lack anything, did you?” They said, “No, nothing.”
Luke 22:36 And He said to them, “But now, whoever has a money belt is to take it along, likewise also a bag, and whoever has no sword is to sell his coat and buy one.
Luke 22:37 “For I tell you that this which is written must be fulfilled in Me, ‘And He was numbered with transgressors’; for that which refers to Me has its fulfillment.”
Luke 22:38 They said, “Lord, look, here are two swords.” And He said to them, “It is enough.”
The means we choose to employ for self-defense (i.e., martial arts, sticks, knives, guns, etc.) are not necessarily important so long as our actions do not go beyond the reasonable needs of self-defense or turn to vengeance, as Paul says:
Rom. 12:17 Never pay back evil for evil to anyone. Respect what is right in the sight of all men.
Rom. 12:18 If possible, so far as it depends on you, be at peace with all men.
Rom. 12:19 Never take your own revenge, beloved, but leave room for the wrath of God, for it is written, “Vengeance is mine, I will repay,” says the Lord.
Paul says our goal is to remain at peace with men so long as the situation depends on our choices and actions. On the other hand, if another breaks the peace with us, then we are justified to act in defense so long as we don’t move to acting in vengeance. Acting in vengeance is initiating revengeful conflict out of malice for another while self-defense is acting without malice to halt or prevent the attack of another. If our actions shift from halting an attack to prosecuting our own attack, then we have crossed the line into sin.
Keep in mind Paul's command for believers to seek for peace with others to the degree possible presumes we will make every effort to avoid potential conflict before it escalates to violence. If we sense a conflict developing around us, we should seek an escape or retreat if reasonably possible. The Bible's acknowledgment of the right to act in self-defense cannot become a license for us to choose a violent response when other, non-violent options were still available.
Finally, as Jesus advised in Luke 22 a Christian may take prudent measures to mitigate potential risks, including taking self-defense courses, lawfully carrying weapons or even acting preemptively against a threat when required to protect life or property. Any actions we take must be in keeping with the laws of our land since Christians are also required to obey and uphold the law.
In the end, the key test for whether we act properly under any circumstances is determined by the motives of our heart. If we act with evil intentions, then we sin; if we act to thwart evil and protect the innocent, then we do not sin.