To above five hundred brethren at once (epanw pentakosioi adelpoi epapax). Epanw here is just an adverb with no effect on the case. As a preposition with the ablative see Matthew 5:14 . This incident is the one described in Matthew 28:16 the prearranged meeting on the mountain in Galilee. The strength of this witness lies in the fact that the majority (oi pleiou) 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.