Compromise. Jesus — specifically the Jesus who in John’s vision wielded a sharp, double-edged sword — saw compromise as a growing threat for the church in Pergamum. Not that the city of Pergamum was subtle. Jesus calls it “Satan’s throne” for a reason. Which is easier to resist as Christians: persecution by enemies (Smyrna) or seduction by culture (Pergamum)? 
Scripture: Revelation 2:12-17