I mentioned in a different post the tricky nature of distinguishing between heresy, heterodoxy, bad teaching, etc. Johannes Wollebius (1586-1629) described some careful thinking and distinguishing between the various categories.
In chapter 27 on “The False Church” of his Prolegomena, he notes:
1. Not every error makes a heretic.
There may be error against the foundation like that of the Arians and Marcionites, who denied, the one the deity, and the other the humanity, of Christ; or concerning the foundation, as the papists err in teaching transubstantiation, by which the truth of the human nature of Christ is taken away; or error by addition to the foundation, which errors are by Paul called hay, wood, etc. (1 Cor. 3:12).
2. The following makes a heretic: (i) an error against the foundation or concerning the foundation, (ii) conviction, (iii) contumacy.
3. Not every schismatic is a heretic.
A schismatic is one who, although holding to the foundation of the faith, departs from some practice [ritus] of the church, rashly or because of ambition.
Brian J. Lund 