What does Revelation 3:15 mean?

Nothing escapes Jesus' attention. He tells the Laodicean church in this verse that He knows its condition. He says it is neither cold nor hot, although He would prefer that it was one or the other. This condemnation of "lukewarm" faith carries dire consequences. In that era, room-temperature water was dangerous water. Cold water suggested a flowing spring or river, and hot water was something which had been cooked or cleaned. Anything in between was, naturally, suspicious and possibly worthless.

The church in Laodicea was neither fervent nor frigid. It was simply content to maintain a status quo position. It was neither emotionally excited about Christian matters nor coldhearted about them. Whereas Jesus offered the church at Philadelphia no reproof, He offered the Laodicean church no praise. Unlike the church at Sardis that had things that remained and needed to be strengthened, the church at Laodicea had nothing that simply needed to be strengthened. In some ways churches exist today that are neither hot nor cold. They simply continue to perform meaningless activities, offer visitors a limp handshake, go through the motions of a sterile liturgy, and keep their members in an indifferent mindset.

Another aspect of Jesus' desire that the church be "either hot or cold" relates to how we respond to the gospel. Those who are "hot" in spiritual matters are deeply involved and committed to faith. Those who are cold, however, are at least in a position where they can be deeply impacted by faith—they can be changed. Those who are "lukewarm" have just enough knowledge of Christ that they're not resistant, but lack enough real faith to be fully engaged. That condition, in truth, is harder to change than to convert a fully-cold non-believer!