heretic

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person who holds controversial opinions, especially one who publicly dissents from the officially accepted dogma of the Roman Catholic Church.
• adjective Heretical.

from The Century Dictionary.

• noun One who holds and persistently maintains an opinion or a doctrine at variance with the accepted standards of any school or party, and rejected or condemned by it; one who rejects a generally accepted belief.
• noun Specifically, in theology, a professed believer who adopts and persistently maintains religious opinions contrary to the accepted standards of his church. See heresy, 2.
• noun Synonyms Heretic, Schismatic, Sectary, Dissenter, Nonconformist. Heretic is an opprobrious epithet for a professed believer who holds religious opinions contrary to the established or dominant beliefs. A schismatic is one who seeks to sunder or divide into different organizations or parties those who are of essentially the same religious faith. A sectary or sectarian is one who sets the welfare of his own sect or denomination above that of the church universal, often pushing its interests at the cost of the general Christian welfare. This word has been much used opprobriously of those who stand out against an original or more powerful organization. A dissenter or nonconformist is one who dissents from an established religion, or does not conform to it; specifically and in actual use these words apply almost exclusively to those Protestants in Great Britain who worship apart from the Established Church of England, as the Presbyterians, Baptists, and Independents.
• Pertaining to heresy; believing heresy.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who holds to a heresy; one who believes some doctrine contrary to the established faith or prevailing religion.
• noun (R. C. Ch.) One who having made a profession of Christian belief, deliberately and pertinaciously refuses to believe one or more of the articles of faith “determined by the authority of the universal church.”

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Someone who, in the opinion of others, believes contrary to the fundamental tenets of a religion he claims to belong to.
• adjective archaic Heretical; of or pertaining to heresy or heretics.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person who holds unorthodox opinions in any field (not merely religion)
• noun a person who holds religious beliefs in conflict with the dogma of the Roman Catholic Church