Why is it judged incredible with you, if for why should it be thought a thing incredible with you, that, A.V.; doth for should, A.V. Why is it judged, etc. The use of d is somewhat peculiar. It cannot stand for ὅτι, but it is nearly equivalent to "whether," as in Acts 26:23.
The question proposed to the mind is here whether God has raised the dead; and in Acts 26:23 whether Christ has suffered, whether he is the first to rise. In the latter case St. Paul gives the answer by his witness to the truth, affirming that it is so.
In the former case he chides his hearers for giving the answer of unbelief, and saying that it is not so.