A.T. Robertson Commentary Acts 26:8

"Why is it judged incredible with you, if God doth raise the dead?" — Acts 26:8 (ASV)

Incredible with you (απιστον παρ' υμιν). This old word απιστον (α privative and πιστος) means either unfaithful (Luke 12:46), unbelieving (John 20:27), or unbelievable as here). Paul turns suddenly from Agrippa to the audience (παρ' υμιν, plural), most of whom were probably Gentiles and scouted the doctrine of the resurrection as at Athens (17:32).

If God doth raise the dead (ε ο θεος νεκρους εγειρε). 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 verse 9 does it in a way.