To above five hundred brethren at once
(epanw pentakosioi adelpoi epapax
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.