'anti-theist' originates from Greek-derived prefix 'anti-' (from Greek 'anti', meaning 'against') combined with 'theist' (from 'theism', ultimately from Greek 'theos' meaning 'god').

'theist' comes from 'theism' which derives from Greek 'theos' ('god'); the compound 'anti-theist' formed in modern English by prefixing 'anti-' to 'theist' to denote opposition, producing the current term 'anti-theist'.

Initially formed simply to label someone 'against theists' or 'against theism', the term has come to denote both a philosophical position rejecting theism and, in some contexts, an active opposition to religious influence.