The ex-priest Tobin accuses the Judge of blasphemy, prompting the Judge to ask a rhetorical question that seems like a fallacy:
What could I ask of you that you’ve not already given?
The rhetorical question appears to be a fallacy because the implied answer (nothing) is false: the Judge could ask the ex-priest for any number of things that Tobin has not yet provided the Judge, be they physical goods or answers to questions. The ex-priest, who specifically refrained from commenting on the Judge's claim that "men of god and men of war have strange affinities," could have instead provided the Judge with a comment on that claim.
However, the ex-priest has become a man of war, joining Glanton's crew and thus implicitly condoning their frequent atrocities. Tobin is then a physical embodiment of the Judge's thesis on man and war. The priest need not say anything, for his life alone is evidence of the Judge's claim that "war is god," which is to say there is nothing more unifying or predictive or significant for the parties at play other than war. There is nothing more determinative of life on earth, nor anything more natural in men themselves.
There is then no answer required by the Judge's rhetorical question: as far as the Judge is concerned, the priest's life as he lives it is all the Judge could ask of him, for it proves that sinning is more natural than faith. When one argues that war is god, an ex-priest living a war-filled life is the best possible proof.