unorthodox — not conforming to rules, traditions, or modes of conduct, as of a doctrine, religion, or philosophy; not orthodox: an unorthodox ideology.
agnostic — An agnostic believes that it is not possible to know whether God exists or not. Compare atheist.
apostate — An apostate is someone who has abandoned their religious faith, political loyalties, or principles.
atheistic — Atheistic means connected with or holding the belief that there is no God.
differing — to disagree in opinion, belief, etc.; be at variance; disagree (often followed by with or from): His business partner always differs with him.
dissenting — to differ in sentiment or opinion, especially from the majority; withhold assent; disagree (often followed by from): Two of the justices dissented from the majority decision.
freethinking — a person who forms opinions on the basis of reason, independent of authority or tradition, especially a person whose religious opinions differ from established belief.
heterodox — not in accordance with established or accepted doctrines or opinions, especially in theology; unorthodox.
iconoclastic — attacking or ignoring cherished beliefs and long-held traditions, etc., as being based on error, superstition, or lack of creativity: an iconoclastic architect whose buildings are like monumental sculptures.
impious — not pious or religious; lacking reverence for God, religious practices, etc.; irreligious; ungodly.
infidel — Religion. a person who does not accept a particular faith, especially Christianity. (in Christian use) an unbeliever, especially a Muslim. (in Muslim use) a person who does not accept the Islamic faith; kafir (def 2).