What does Revelation 3:1 mean?

Write this to the messenger of the church in Sardis: The One who holds the seven spirits of God and the seven stars says, I know what you’ve been doing. People think you’re alive, but really, you’re dead inside. Revelation 3:1 - Modern Text Bible

(Jesus is speaking to the church in Sardis, calling out their reputation for being spiritually alive, but exposing their true spiritual deadness.)