False witnesses of God (ψευδομαρτυρες του θεου). Late word, but ψευδομαρτυρεω, to bear false witness, old and common. The genitive (του θεου) can be either subjective (in God's service) or objective (concerning God). Either makes good sense.

Because we witnessed of God (οτ εμαρτυρησαμεν κατα του θεου). Vulgate has adversus Deum. This is the more natural way to take κατα and genitive, against God not as equal to περ (concerning). He would indeed make God play false in that case, if so be that the dead are not raised (ειπερ αρα νεκρο ουκ εγειροντα). Condition of first class, assumed as true. Note both περ intensive particle indeed and αρα inferential particle therefore .