What does Revelation 2:16 mean?

In this verse Jesus commands the church at Pergamum to change its mind about the teachings of Balaam and the Nicolaitans. If there is no repentance, Jesus will visit the church soon—in this context, meaning "suddenly"—and fight against the unrepentant members. He will judge them with the sword of His mouth, the Word of God.

God does not tolerate false doctrine and immorality in the church. Christians who try to harmonize biblical teaching and godly living with corrupt doctrine and immoral living learn the hard way that the two do not mix. They place themselves under swift chastisement at the hand of Jesus, the Head of the Church. Instead of rebuking the moral and spiritual darkness that filled Pergamum, the church at Pergamum compromised its purity by allowing the teachings of Balaam and the Nicolaitans to fester in the assembly. False teaching has a greater capacity to damage a church from within than from the outside.