In Luke 22:36-38 Jesus tells his disciples to buy a sword. Using this as a prooftext for self-defense seems to miss the point (cf. Matthew 16:6-7, John 6:52). This seems like a case of “right idea, wrong prooftext.”
Consider Jesus’ response, “It is enough!” (v. 38) Enough for what? Not in this case enough to defend Jesus from those coming to arrest him.
Jesus later heals the ear of the man Peter sliced, and says:
“Put your sword back into its place. For all who take the sword will perish by the sword.” (Matthew 26:52)
In the text itself Jesus explains why he tells them to bring a moneybag, a knapsack, and a sword:
“For I tell you that this Scripture must be fulfilled in me: ‘And he was numbered with the transgressors.’ For what is written about me has its fulfillment.” (Luke 22:37)
Not for self-defense. But to fulfill prophecy.
Jesus is probably also speaking ironically: Before they were told, “Carry no moneybag, no knapsack, no sandals, and greet no one on the road.” (Luke 10:4, cf. Matthew 10:9-10). The situation has changed. The opposition has escalated. Resistance and death are forthcoming. Jesus did not want his disciples to physically protect him. Jesus was following the script, being “numbered with the transgressors.” Jesus was embracing martyrdom, as would the apostles later.
While there is an appropriate time for public charity, public prayer, public fasting, taking oaths, and self-defense, the Christian life is chiefly marked by a general inner disposition to practice secret righteousness, secret prayer, secret fasting, avoidance of oaths, non-resistance and non-violence (Matthew 5-6).