This is where we must draw the line of distinction - DOCTRINAL ERROR is the misinterpretation or application of any biblical teaching. DAMNING HERESY is any doctrine or practice that is contrary to salvation by grace alone, through faith alone, in Christ alone. Obviously, any teaching that allows the worship of false gods, denies the deity of Christ, his virgin birth, or vicarious atonement is damning heresy. But there are other forms of heresy far more subtle and dangerous.
To teach sprinkling for baptism is grave error, but not necessarily damning. However, to make baptism a condition of grace to any degree is damning heresy. To substitute grape juice for wine in the Lord's Supper is serious error, though not damning to the soul. But to make the Lord's Supper a sacrament by which grace is conferred upon a sinner is damning heresy. The list could be greatly enlarged. But there is one test by which damning heresy can always be identified. ANY DOCTRINE THAT TEACHES, OR PRACTICE THAT LEADS PEOPLE TO BELIEVE, THAT SALVATION IS IN ANYWAY CONDITIONED UPON, DEPENDENT UPON, OR DETERMINED BY THE WORTH, WORK, OR WILL OF THE SINNER IS DAMNING HERESY AND ALWAYS DEADLY TO THOSE WHO EMBRACE IT. Arminian, free-will, works religion is as damning to the souls of men as Judaism, Islam, Catholicism, or satan worship. It is our responsibility to expose heresy by instructing men in the truth and to reject those who will not obey the truth as self-condemned heretics (Tit.3:9-11).