John Gill Commentary Revelation 2:16

"Repent therefore; or else I come to thee quickly, and I will make war against them with the sword of my mouth." — Revelation 2:16 (ASV)

Repent
This is said to those who were truly godly in those times, but still retained their communion with these corrupt men, and had not, as yet, separated from them; nor had they protested against these evil doctrines and practices, at least but very coldly, and had too much connived at them; and therefore are called upon to repent of their lukewarmness, negligence, and sinful compliances:

or else I will come unto you quickly :
in a providential way, to rebuke and chastise for such remissness, indifference, and evil communication:

and will fight against them with the sword of my mouth ;
of this sword, (See Gill on Revelation 1:16). And it is observable, that Christ does not say, "I will fight against you"; the true members of his mystical body, though lax and supine, for Christ does not fight against his people, but for them; but he says, "and will fight against them"; the Balaamites and Nicolaitans, for their idolatry and uncleanness.