Any doctrine that deviates from the historical, orthodox, and biblical position of the Christian Church, throughout Church history, as judged from a Protestant perspective is a heresy. There are heresies that are damnable (denying the Deity of Christ, denying Christ’s physical resurrection, denying justification by grace through faith, etc.). There are heresies that are not damnable (advocating women pastors; practicing polygamy, divorce for convenience sake, etc., although the repentance is necessary). There are also teachings within Christianity that are debatable whereas differences of opinion are not heresy (eating or not eating meat, worship on Saturday or Sunday, etc.) See Rom. 14:1-12, and Col. 2:16-19