A casual reading may seem to prove these verses a little strange. These verses appear to be discussing aspects about Jesus in His Second Coming. This is because it describes taking vengeance upon nonbelievers. It sort of reads like Jesus coming back in His role to rule over the world.
Here's the problem: Notice the word "their" in verse 6. Who is "their"? Based on the previous few verses, it is talking about believers. My point is these verses are describing believes with swords in their hands to carry out God's vengeance.
The reference to the sword has to do with believers using weapons for God's glory. Let me explain, God gives the privilege to believers to lead others to Him. It also appears that based on these verses, God uses believers in order to take out His vengeance on those who don't trust in Him.
The classic example is when the Israelites first conquered the Promised Land as taught in the book of Joshua. God used the Israelites to judge those who lived there. My point is God uses His people in order to carry out judgment. If that is true, when do we punish nations and put their leaders (kings) in fetters? Yes, the Israelites have conquered enemies throughout their history, but how does this apply to us? The answer comes back to the concept that evil always lose in the end. Sometimes evil men and armies win battles, but eventually those who stand up for Him eventually conquer over those who oppose Him. God may use whatever and whomever He chooses to judge nonbelievers.
The point is in effect that God's will, will get done, through people. No matter how bad a situation may look in our lives or in the life of a society, God finds a way for His people to win out if collectively, they are putting their trust in Him.