{To above five hundred brethren at once} (epan(930a)pentakosiois adelphois ephapax). Epan(935c) 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 (hoi
pleious) 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.