unbeliefEnglishOxford American Thesaurusunbelief noun a symptom of unbelief was an inability to pray: atheism, nonbelief, agnosticism, heresy, apostasy, irreligion, heathenism, godlessness, nihilism, lack of faith; skepticism, cynicism, disbelief, incredulity, suspicion, distrust, mistrust, doubt, nonconviction, dubiety. ▷antonyms belief, faith.