Ephapax

For in that he died, he died unto sin once:

but in that he liveth, he liveth unto God.

                                                                   Romans 6:10

"The Greek word ephapax translated “once” in this verse actually means “once for all.” Christ did not have to die again and again, a new death for every sinner. He died unto sin once for all, His death being sufficient to take away “the sin of the [whole] world” (John 1:29).

The word ephapax occurs only five times in the Bible.

--Our text is the first, confirming that His once-for-all death for sin was sufficient forever; He now lives wholly “unto God.”

--The second confirms the reality of this permanent resurrection. In Jewish law, a factual claim was considered confirmed by the principle that “in the mouth of two or three witnesses every word may be established” (Matthew 18:16). 

--The other three references are in Hebrews: “[He] needeth not daily, as those high priests, to offer up sacrifice, first for his own sins, and then for the people’s: for this he did once [that is, ‘once for all’], when he offered up himself.” “Neither by the blood of goats and calves, but by his own blood he entered in once [‘once for all’] into the holy place, having obtained eternal redemption for us.” “By the which will we are sanctified through the offering of the body of Jesus Christ once for all” (Hebrews 7:27; 9:12; 10:10).

Once for all He died for sin, then with His own shed blood He entered into the presence of the Father, sanctified us forever, and was raised from the dead by impeccable testimony, once for all." HMM