The term, 'lukewarm', is used in reference to the church of Laodicea in Revelation 3.
It helps to know that the city of Laodicea was wealthy because of the wool it manufactured and sold, and for an eye salve developed by the medical school there.
But they did not have a water source of their own. They had hot water piped down from Hierapolis (location of a hot springs) and cold water from Colossae. By the time it got to Laodicea, it was lukewam. Hot water was good for healing purposes, and cold for drinking to relieve thirst. But lukewarm is not so useful.
Now look at what is said to them:
Rev 3:14 "To the angel of the church in Laodicea write: The Amen, the faithful and true Witness, the Beginning of the creation of God, says this:
Rev 3:15 'I know your deeds, that you are neither cold nor hot; I wish that you were cold or hot.
Rev 3:16 'So because you are lukewarm, and neither hot nor cold, I will spit you out of My mouth.
Rev 3:17 'Because you say, "I am rich, and have become wealthy, and have need of nothing," and you do not know that you are wretched and miserable and poor and blind and naked,
Rev 3:18 I advise you to buy from Me gold refined by fire so that you may become rich, and white garments so that you may clothe yourself, and {that} the shame of your nakedness will not be revealed; and eye salve to anoint your eyes so that you may see.
Rev 3:19 'Those whom I love, I reprove and discipline; therefore be zealous and repent.
Rev 3:20 'Behold, I stand at the door and knock; if anyone hears My voice and opens the door, I will come in to him and will dine with him, and he with Me.
Rev 3:21 'He who overcomes, I will grant to him to sit down with Me on My throne, as I also overcame and sat down with My Father on His throne.
The church believes they are rich, but they are poor. Perhaps they are putting faith in how well they are doing financially rather than tending to their spiritual needs.
And look at how Jesus is shown outside, knocking, offering to come in to anyone who opens the door.
Laodicea is a money rich, spiritually poor church that does not have Christ. They may claim to be Christian, but they aren't listening to Christ and He is outside. They were wealthy because of their eye salve and wool...but they were spiritually blind and naked.
So to describe today's lukewarm Christians - it is those who claim Christ, but are spiritually weak, don't grow, don't reach others, are not a witness in how they live their lives. They may be in a wealthy church, but they, themselves, are not filled with the Spirit and need Christ in their lives.