You would be in error, but you would not necessarily belong to a false religion. If you belong to a church that is Trinitarian and confesses Christ as the Son of God incarnate--i.e., it confesses the Nicene Creed--yet that church is in error on the efficacy of baptism, your church would be considered heterodox rather than heretical. This is not to excuse or condone error, though. However, the JWs and the Mormons do not even get God right, so they are considered outside the scope of Christendom.