Vincent's NT Word Studies
4. Beareth (forei). Beareth and weareth. A frequentative form of ferw to bear.Sword (macairan). See on Apoc. vi. 4. Borne as the symbol of the magistrate's right to inflict capital punishment. Thus Ulpian: "They who rule whole provinces have the right of the sword (jus gladii)." 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."
Robertson's NT Word Studies
13:4 {A minister of God} (qeou diakonos). General sense of diakonos. Of course even Nero was God's minister "to thee (soi ethical dative) for good (eis 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 poiis). Condition of third class, ean and present active subjunctive of poiew, "if thou continue to do." {Sword} (macairan). 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} (ekdikos). Old adjective from ek and dike (right), "outside of penalty," unjust, qen in later Greek "exacting penalty from one," in N.T. only here and #1Th 4:6.