John Gill Commentary Revelation 3:16

"So because thou art lukewarm, and neither hot nor cold, I will spew thee out of my mouth." — Revelation 3:16 (ASV)

So then because you are lukewarm, and neither cold nor hot ,
&c.] A lukewarm professor is one that serves God and mammon; that halts between two opinions, and knows not what religion is best, and cares little for any, yet keeps in a round of duty, though indifferent to it, and contents himself with it; and is unconcerned about the life and power of godliness, and takes up with the external form of it; and has no thought about the glory of God, the interest of Christ and truth; and this was too much the case of this church, at least of a great number of its members; therefore it was very loathsome to Christ, hence he threatens:

I will spit you out of my mouth ;
this shows how nauseous lukewarmness is to Christ, insomuch that on account of it he would not own and acknowledge her as his; but even cast her out, unchurch her, and have no more any such imperfect church state upon earth, as he afterwards never will, this is the last; nor is there any church state, or any remains of one in Laodicea; it is indeed quite uninhabited.