Outwardly, many members in Sardis bore Christ's name, but like the apparently religious Pharisees, they were in reality hypocrites, full of dead men's bones.
Satan champions self-opinion and mob behavior. God's church has never been immune to division because each member has only part of the truth.
Most of the attrition from the truth stems from losing interest. Drifting away is rarely intentional, but the result of choosing to live carnally.