A Luke 22:35-38 reads in context: “And He said to them, `When I sent you without money bag, knapsack, and sandals, did you lack anything?’ So they said, `Nothing.’ [God took care of them.] Then He said to them, `But now, he who has a money bag, let him take it, and likewise a knapsack; and he who has no sword, let him sell his garment and buy one. For I say to you that this which is written must still be accomplished in Me: “And He was numbered with the transgressors.” For the things concerning Me have an end.’ So they said: `Lord, look, here are two swords.’ And He said to them: `It is enough.'”
This passage cannot be used to justify participation of a Christian to fight in war. In fact, the passage teaches the exact opposite. First of all, “two swords” would hardly be enough for Christ’s disciples to defend themselves against the coming Roman persecution. Secondly, Christ Himself makes clear why they were to buy swords. It was so that the prophecy regarding Him could be fulfilled. What specific prophetic saying had to be fulfilled? “And He was numbered with the transgressors.” What transgression did the disciples – who had swords – become guilty of?