The Greek adverb hapax means once, one time, on one occasion. When used with the definite article or in theological contexts, it carries the sense of once for all — a singular, unrepeatable event that has permanent, decisive effect. It is related to ephapax (G2178, once for all/at once).
The hapax theology of Hebrews is among the most powerful arguments for the finality and sufficiency of Christ's atonement. Where the Levitical high priest entered the Holy of Holies once a year (Hebrews 9:7), Christ entered the true heavenly sanctuary once for all (Hebrews 9:12, 26; 10:10). The repetition of animal sacrifices implicitly confessed their inadequacy; Christ's single, decisive sacrifice implicitly proclaims its eternal perfection. Jude 3 uses hapax for the faith 'once for all delivered to the saints' — the completed revelation requiring no additions. The word is a theological safeguard against both Catholic-style re-sacrifice and modern additions to the gospel.