Romans 13:4
A minister of God (qeou diakonov). General sense of diakonov. Of course even Nero was God's minister "to thee (soi ethical dative) for good (eiv to agaqon, for the good)." That is the ideal, the goal.
Beareth (forei). Present active indicative of forew, old frequentative form of ferw, to bear, to wear.
But if thou do (ean de poihv). Condition of third class, ean and present active subjunctive of poiew, "if thou continue to do."
Sword (maxairan). Symbol of authority as to-day policemen carry clubs or pistols. "The Emperor Trajan presented to a provincial governor on starting for his province, a dagger, with the words, 'For me. If I deserve it, in me'" (Vincent).
An avenger (ekdikov). Old adjective from ek and dikh (right), "outside of penalty," unjust, then in later Greek "exacting penalty from one," in N.T. only here and I Thessalonians 4:6.
