Greek ephapax (a strengthened form of hapax, "once"). Used seven strategic times in the NT, the word carries two meanings at once: unrepeatable in time (one historical act) and definitive in effect (no successor act needed). Paul uses it for Christ's death (Romans 6:10) and resurrection appearances (1 Corinthians 15:6). Hebrews hammers it repeatedly for the single, sufficient sacrifice that ends the sacrificial system (Heb 7:27, 9:12, 9:26, 10:10). Jude applies it to "the faith once for all delivered to the saints" (Jude 3).
"Once for all" is the death blow to every system that requires repeated atoning work — the Catholic Mass as re-sacrifice, re-baptism after grave sin, working off one's salvation by penance or merit. "By a single offering He has perfected for all time those who are being sanctified" (Hebrews 10:14). Past tense perfected; ongoing present being sanctified. The position is settled once; the growth into it takes a lifetime. Same word also fences off the content of the faith: Jude did not write "the faith progressively revealed to each generation" or "the faith contextualized anew." It was delivered once for all, and we are to contend for it. Tradition that adds to apostolic deposit violates ephapax; progressive theology that subtracts from it violates ephapax. The Christian faith is not a franchise that updates its menu.