Rhetorical: Sherem is surely educated in the art of public argument. Rather than the simple answer, he gives a complicated one. the simple answer would be "Yes." In this case, however saying a simple yes to the question places too much power in Jacob's hands. What Sherem does is turn the question to his own advantage.
Sherem realizes that while he may deny Christ, saying it in precisely that way assumes the reality, and Sherem would be seen as denying common presumption. Rather than risk that, Sherem cleverly indicates "No." In this way anyone not following the argument closely would not perceive a great gulf between the two men (though it is clearly there). Also, Sherem makes sure that everyone understands that he cannot deny the existence of something that does not exist. The has the effect of turning the argument back to Jacob to require proof which might compel belief. Sherem exudes confidence tat such proof cannot be found.