Heretic, schismatic, sectarian, sectary, dissenter, nonconformist are comparable when denoting a person who from the point of view of a particular church or religious faith is not orthodox in his beliefs.
Heretic applies to one who teaches and maintains doctrines that are contrary to those which are actually taught by the church or faith to which he belongs or has belonged.
Schismatic applies to one who separates from or provokes division in a church or communion usually by differing on a minor point or points of doctrine; thus, from the point of view of the Roman Catholic Church, those Eastern Christians who seceded to form the Orthodox Church are schismatics, whereas Luther, Calvin, Cranmer, and other leaders of the Reformation are heretics: to the Church of England, the early Puritans and Quakers were schismatics.
Sectarian may be applied to a member of a religious denomination or sect, often neutrally but sometimes with the implication of a rigorous and bigoted adherence.
Sectary, which is chiefly historical, more than sectarian, implies membership in a sect that is relatively small and composed of ardent and often by connotation narrow-minded and bigoted partisans.
Dissenter, which basically means one who dissents, in the present connection, applies to a person who separates himself from and worships in a communion other than an established church (as the Church of England); nonconformist is ordinarily synonymous with dissenter, but the term has been specifically applied in England to persons who refused to accept certain religious doctrines or to follow certain religious practices imposed by the established church; thus, many of the 2000 clergymen who refused to subscribe to the Act of Uniformity in 1662 were regarded as nonconformists; Roman Catholics in England (as a class) have been held to be nonconformists rather than dissenters, since they did not accept the Church of England at any time. Nevertheless the terms are often used interchangeably.