A.T. Robertson Commentary 1 Corinthians 15:6

"then he appeared to above five hundred brethren at once, of whom the greater part remain until now, but some are fallen asleep;" — 1 Corinthians 15:6 (ASV)

To above five hundred brethren at once (επανω πεντακοσιοις αδελφοις εφαπαξ). Επανω here is just an adverb with no effect on the case. As a preposition with the ablative see Mt 5:14. This incident is the one described in Mt 28:16 the prearranged meeting on the mountain in Galilee. The strength of this witness lies in the fact that the majority (ο πλειους) of them were still living when Paul wrote this Epistle, say spring of A.D. 54 or 55, not over 25 years after Christ's resurrection.