Which church was lukewarm — neither hot nor cold?

Answer: Laodicea

Because Laodicea was lukewarm, neither hot nor cold, Jesus said He was about to vomit it out of His mouth.

“To the angel of the church in Laodicea write: These are the words of the Amen, the faithful and true Witness, the Originator of God’s creation. I know your deeds; you are neither cold nor hot. How I wish you were one or the other! So because you are lukewarm—neither hot nor cold—I am about to vomit you out of My mouth!”— Revelation 3:14-16 (BSB)