What does Revelation 3:16 mean?

In this verse Jesus describes the church at Laodicea as lukewarm. It was customary for people in ancient times to drink hot or cold beverages at their feasts and in their religious sacrifices, but they never drank a lukewarm beverage. Such liquids are distasteful, and for good reason: they are more likely to harbor diseases. Today, on a hot day, people find a cold drink refreshing, and on a cold day they find a hot drink invigorating, but no one ever enjoys a lukewarm drink.

Laodicea did not have its own water source. It depended upon the nearby city of Hierapolis for its water. Hierapolis was constructed around numerous hot springs, so it enjoyed an abundant supply of hot water that it sent to Laodicea by way of an aqueduct. However, by the time the water reached Laodicea it had cooled to a lukewarm temperature. It would need to be cooled or re-heated prior to being fit for consumption. The lukewarm taste of the Laodiceans' religious life made Jesus feel so sick He felt like vomiting the church out of His mouth.

This presents a useful analogy for evangelism, as well. Those who are spiritually "hot" are engaged in their faith. Those who are "cold" have an opportunity to be influenced in a powerful way by the gospel. But those who are "lukewarm" are actually in a worse condition that those who are "cold." They know "just enough" about Jesus, so they're not resistant, but they're also somewhat calloused to His voice. From Jesus' perspective, it's actually better to be spiritually "cold," since that means you're more likely to notice the calling of God.