Albert Barnes 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, I will spew you out of my mouth. Referring, perhaps, to the well-known fact that tepid water tends to produce sickness at the stomach, and an inclination to vomit. The image is intensely strong, and denotes deep disgust and loathing at the indifference which prevailed in the church at Laodicea. The idea is that they would be utterly rejected and cast off as a church: a threatening of which there has been an abundant fulfillment in subsequent times. It may also be remarked that what was threatened to that church may be expected to occur to all churches if they are in the same condition, and that all professing Christians and Christian churches that are lukewarm have special reason to dread the indignation of the Savior.