1. Luke explicitly says that Jesus told his disciples to get swords to fulfill prophecy - that he would be numbered with the transgressors. When Christ was met by officals in the garden, he could be numbered a transgressor (or a political anarchist, maybe like a zealot) because his group was obviously armed. Verse 52 of the chapter adds to this plausibility when Jesus says, “Am I leading a rebellion, that you have come with swords and clubs?" Luke tells us that the reason Jesus had his disciples get swords was to be considered a transgressor and says nothing about considering them for defense.
2. When the disciples come up with a whopping two swords, Jesus says that it is enough. It certainly wasn't enough to consider for self-defense, but it was enough to be numbered with the transgressors. The number two may have some significance to bolster this notion. In Jewish law, one could not be convicted of a crime based on one witness. So if there would have been only one sword present, it may not have been enough of a witness or evidence to hold a charge against Jesus.
3. When Peter uses his sword in Christ's defense, Jesus tells him to sheath it. Some of the early church fathers took this command as an example of Christ's command for all Christians to put away violence (see section 4). The fact that none of the apostles ever (to our knowledge) unsheathed their swords against anyone again seems to indicate to me that Christ was making more than a momentary statement. In light of the evidence of example and early church interpretation, it seems that such a conflicting notion implies that this more ambiguous notion must be explained in another way than that Christ was advocating violence.
4. Compare Luke 22:36 in the betrayal scene with Luke 9:3 when Jesus sent out the disciples to spread the good news of the Kingdom.
“But now if you have a purse, take it, and also a bag; and if you don’t have a sword, sell your cloak and buy one."
“Take nothing for the journey—no staff, no bag, no bread, no money, no extra shirt.
In Luke 9, Christ's servants were not to worry about money, about possessions (a bag), and they were to have only the clothes on them (but no extra undershirt). In Luke 22 we see a complete reversal. Now Jesus says that the disciples should take their money and possessions, but sell their outer garments (their main article of clothing) to purchase a weapon - a weapon Jesus will condemn them for using in just a few moments. There is obviously something weird going on here. This becomes even more apparent when you see Christ's words in Luke 22:35, immediately preceding him telling the disciples to buy swords. Jesus refers to the command of Luke 9 to take no money or possessions and asks the disciples if they lacked anything. The disciples answered that they had not lacked anything. God provided and was sufficient. So if they didn't need anything but the clothes on their back, how had God changed now that would require them to take money, possessions, and a sword?
What we see in Christ's words found in Luke 22 is that he is predicting the betrayal of the apostles (see video at the bottom for thorough explanation). Just as he told Peter that he would betray Jesus in word, now he is telling the apostles that they will betray him in action. Immediately prior to this conversation, the disciples had been fighting over who would be greatest in the Kingdom, and Christ said that it was the one who serves. Then he goes on into the betrayal texts. It's as if Jesus was saying,
hey, were you guys lacking anything when I sent you out to proclaim the Kingdom? No? Well, actually, I think you were. Do the exact opposite of what I told you to do before, and pick up some weapons while you're at it. Oh, what? You already have two swords on you? Where did those come from and why did you feel the need to add to what I told you was sufficient before? I guess Isaiah was right. This fulfills the prophecy. I am numbered with the transgressors.
This short explanation of the text really does the beauty of the narrative injustice. I'll reference this narrative again in my final point and provide some links to hear this passage explained in much more detail. I'd encourage you to follow up on those resources.
5. There are many aspects of the Lucan account here that don't add up as many normally interpret it. We import a lot of ideology here rather than allowing Luke to tell his story. As one example, where the passage says that Jesus was to be numbered with the transgressors, many think this refers to the thieves on the cross. In the immediate context, however, that interpretation is obviously wrong. We import that interpretation.
When you look at Luke's narrative of the two swords and the surrounding events, his message begins to pop out. Luke diverges from Matthew and Mark in his use of Isaiah 53 for reference. Luke is the only gospel that mentions the healing of the servant's ear. He's the only gospel that references an angel ministering to Christ in the garden. And the list goes on. Luke's main story is centered around the suffering servant, and he displays how this servant was left on his own by the betrayal of Peter with his words, and the disciples by their actions.
I understand this is a very insufficient explanation, but it would require pages to parse this whole concept out. If you're interested in discovering the beauty of Luke's writing in this narrative, you really need to check out the Naked Bible podcast on this issue (or watch the video below).