Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class VI. Words Relating to the Sentient and Moral Powers
Section V. Religious Affections
3. Religious Sentiments

989. Irreligion.

   NOUN:IRRELIGION, indevotion, impiety; ungodliness &c. adj.; laxity, apathy, indifference; quietism, passiveness, passivity.
  SKEPTICISM, doubt; unbelief, disbelief, incredulity, incredulousness &c. adj.; want of -faith, – belief; pyrrhonism; doubt [See Unbelief. Doubt]; agnosticism, freethinking; deism; hylotheism; materialism, rationalism, positivism; nihilism.
  INFIDELITY, antichristianity, antichristianism, atheism.
  UNBELIEVER, infidel, atheist, antichristian; giaour [Turk.], heretic, miscreant [archaic], heathen, alien, gentile, Nazarene; esprit fort [F.], freethinker, skeptic, pyrrhonist, deist, latitudinarian, rationalist; materialist, positivist, nihilist, agnostic, somatist, theophobist.
   VERB:BE IRRELIGIOUS &c. adj.; disbelieve, lack faith; doubt, question 7c. 485.
  dechristianize, antichristianize [rare]; serve Mammon, contend against the light, love darkness rather than light, deny the truth.
   ADJECTIVE:IRRELIGIOUS; indevout, undevout, devoutless, godless, graceless, ungodly; unholy, unsanctified, unhallowed; atheistic, without God.
  SKEPTICAL, freethinking, unbelieving, unconverted; incredulous, faithless, lacking faith; deistic, deistical; antichristian, unchristian.
  WORLDLY, mundane, earthly, carnal; worldly &c. worldly-minded, unspiritual.
  1. Unbelief is blind.—Milton
  2. The fool hath said in his heart, There is no God.—Bible
  3. No one is so much alone in the world as a denier of God.—Richter