One time (2178) (ephapax from epi = upon, at + hapax = once, a compound of "ha-" [="heis" in compounds] and "pax" [pegnumi = make firm, bring together] = giving hapax the fundamental meaning of numerical singularity and completeness which needs no additions) means once and for all or all at once. At the same time (all together) as here in 1Co 15:6. Friberg says that ephapax is used as a religious technical term for the uniqueness and singularity of the Christ's death and the resultant redemption once (and) for all (Heb 10:10) (Friberg, T., Friberg, B., & Miller, N. F. Analytical Lexicon of the Greek New Testament. Baker Academic) Ephapax - 5x in 5v - Ro 6:10; 1Cor 15:6; Heb 7:27; 9:12; 10:10. As you observe these passages below, notice that all except for 1Co 15:6 relate to the sacrificial death of Christ. NAS = once for all(4), one time(1). There are no uses in the Septuagint. Romans 6:10-note For the death that He died, He died to sin (see hamartia) once for all; but the life that He lives, He lives to God. Comment: When Jesus cried out "It is finished!" (Jn 19:30-note), He was issuing a shout of victory testifying that once and for all time and eternity, the power of sin had been defeated by His work on the Cross, a work which was "validated" by His subsequent resurrection. Ephapax also points out the uniqueness of Christ's death. Vine: Ephapax, “once for all,” once and completely, to be distinguished from pote, “once upon a time.” It is a strengthened form of hapax, which has the same significance, “once for all,” and is used with reference to the death of Christ in He 9:26-note, He 9:28-note; 1Pe 3:18-note. The word in this respect marks the absolute sufficiency and finality of the death of Christ for all the purposes for which He died. (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson or Logos) 1Corinthians 15:6-note After that He appeared to more than five hundred brethren at one time (Here the idea of ephapax is while they were all together as opposed to separately), most of whom remain until now, but some have fallen asleep; Hebrews 7:27-note who does not need daily, like those high priests, to offer up sacrifices, first for His own sins and then for the sins of the people, because this He did once for all (See Ro 6:10 comment above) when He offered up Himself (. Hebrews 9:12-note and not through the blood of goats and calves, but through His own blood, He entered the holy place once for all, having obtained eternal redemption. Comment: Jesus our Great High Priest fulfilled the "shadow" of the Day of Atonement (Lev 16:3, 15, 30 - notice that the Jewish high priest had to enter at least twice, while Jesus entered only once for He only needed one sacrifice -- Himself, the perfect Lamb of God, Jn 1:29) in which the Jewish high priest entered the holy of holies as a representative for the Jewish people. Notice that by entering thus into the Holy of Holies, the Messiah obtained eternal redemption for lost sinners - another truth to substantiate the surety of the eternal security for all who are genuinely by grace through faith in Christ. Believers cannot lose "eternal redemption"...now or ever! Hebrews 10:10-note By this will we have been sanctified (perfect tense = Set apart at a point in time = at the Cross [specifically all "we" of all the ages who are believers = we who were "co-crucified" with Christ - Ro 6:6-note], the effect and power of which stands or remains) through the offering (1Pe 2:24-note) of the body of Jesus Christ once for all. (Note the superiority of the New Covenant and our Great High Priest's once for all sacrifice which stands in stark contrast to the daily sacrifices that the high priests were required to offer under the Old Covenant, the Mosaic Law.) These five hundred brethren would be part of the "many infallible proofs" (Acts 1:3) that make this the greatest event in history since the creation. Ray Stedman notes that... Every generation, the theory is propounded that Jesus really did not rise from the dead physically, that the disciples were so caught up in the wonder of his personality, that they so wanted him back they actually hallucinated and imagined they saw him. But this event, of course, can hardly fit that category, for here there were over five hundred individuals. Now it is hard enough to get one person to hallucinate, but to get five hundred people from various backgrounds and attitudes, etc., to do so all at once is simply incredible. I think this occurred up on a mountainside in Galilee, for even before his crucifixion the Lord had said that he would meet his disciples in Galilee after the resurrection. The first message he sent by the women at the tomb was, "go and tell my brethren to go to Galilee, and there they will see me," {cf, Matt 28:10}. Now you can imagine that word of that spread rapidly throughout the whole believing community and everybody who could get away headed for Galilee. Who would have wanted to miss that most exciting of all Christian meetings? So it is no wonder there were five hundred or more waiting for him on the mountainside, and to them he appeared. (1 Corinthians 15:5-11 They Saw Him Alive) MOST OF WHOM REMAIN UNTIL NOW, BUT SOME HAVE FALLEN ASLEEP: ex on oi pleiones menousin (3PPAI) eos arti, tines de ekoimethesan; (3PAPI): (1Cor 15:18; Acts 7:60; 13:36; 1Thessalonians 4:13,15; 2Peter 3:4) Most of whom remain - Paul's point is that even though 1Corinthians was written more than 25 later, most of the witnesses of the Risen Christ were still alive. MacArthur comments that The quality of specific witnesses is represented by the apostles, all of whom were known by name and could easily be questioned. The quantity of witnesses is seen in the five hundred brethren who all saw the risen Christ at one time. Scripture gives no indication of who those people were, or where Jesus appeared to them, but they were surely well known in the early church, and, like the twelve, would often have been questioned about seeing the risen Savior. (MacArthur, J: 1Corinthians. Chicago: Moody Press or Logos) Remain (3306) (meno) means to abide, to continue to be present, and in context to survive or live. In the present tense. 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