Incredible with you (apiston par' humin). This old word apiston (a privative and pistos) means either unfaithful (Luk 12:46), unbelieving (Joh 20:27), or unbelievable as here). Paul turns suddenly from Agrippa to the audience (par' humin, plural), most of whom were probably Gentiles and scouted the doctrine of the resurrection as at Athens (Act 17:32).
If God doth raise the dead (ei ho theos nekrous egeirei). Condition of the first class assuming that God does raise dead people. Only God can do it. This rhetorical question needs no answer, though the narrative resumed in Act 26:9does it in a way.