Jamieson, Fausset and Brown on Acts 26:8

8. Why should it be thought a thing incredible ... that God should raise the dead?--rather, "Why is it judged a thing incredible if God raises the dead?" the case being viewed as an accomplished fact. No one dared to call in question the overwhelming evidence of the resurrection of Jesus, which proclaimed Him to be the Christ, the Son of God; the only way of getting rid of it, therefore, was to pronounce it incredible. But why, asks the apostle, is it so judged? Leaving this pregnant question to find its answer in the breasts of his audience, he now passes to his personal history.