Repent therefore (metanoēson oun). First aorist (tense of urgency) active imperative of metanoeō with the inferential particle oun (as a result of their sin).
I come (erchomai). Futuristic present middle indicative, “I am coming” (imminent), as in Rev 2:5 with tachu as in Rev 3:11; Rev 11:14; Rev 22:7, Rev 22:12, Rev 22:20. As with en tachei (Rev 1:1), we do not know how soon “quickly” is meant to be understood. But it is a real threat.
Against them (met' autōn). This proposition with polemeō rather than kata (against) is common in the lxx, but in the N.T. only in Rev 2:16; Rev 12:7; Rev 13:4; Rev 17:14 and the verb itself nowhere else in N.T. except Jam 4:2. “An eternal roll of thunder from the throne” (Renan). “The glorified Christ is in this book a Warrior, who fights with the sharp sword of the word” (Swete).
With (en). Instrumental use of en. For the language see Rev 1:16; Rev 2:12; Rev 19:15.