“Romans 13”-Quoting-Christians do so oblivious to the key context of the that text: it is addressed to Christians who are called to fear the sword of God-instituted government.
“But if you do wrong, [Christians], be afraid, for he does not bear the sword in vain.”
Paul has just asked these Roman Christians:
“Would you have no fear of the one in authority? Then do what is good…”
The key backdrop, standing behind these verses, involves the Zealots, the 1st Century insurrectionists who picked up the sword against Roman soldiers whenever they found an opportune moment. They sought to overthrow their occupiers, their Roman enemy, and they would bring down the house with the Jewish War which would begin in A.D. 66.*
Paul writes to the Roman Christians in the Year of Our Lord 57, three years after the Jews began their return to Rome following the death of Claudius, who had expelled the Jews from Rome c. A.D. 49, following the insurrections of A.D. 46-48, and the execution of the sons of Judas the Galilean.
Judas had led an insurrection (Acts 5:37) in A.D. 6 when Judea became a Roman Province. Varus crushed that revolt with the sword, and crucified 2,000 Jews around Jerusalem (see Josephus).
In this pain-filled milieu, Paul writes to the Roman Christians, Jew and Gentile:
“Do not be conformed to this world…Bless those who persecute you…Do not repay anyone evil for evil…Do not take revenge…If your enemy is hungry, feed him…Do not be overcome by evil, but overcome evil with good. Everyone must submit himself to the governing authorities…”
*A fuller , annotated timeline and text, here.
TextsInContext 
Thank you for highlighting this context. It speaks volumes to Christians of all eras about our real duties before God and society. We are to be nonviolent peacemakers and must shun insurrectionist movements of all kinds. How relevant to our own times!
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