Hm. I'll go with my first gut reaction. It's probably more in-character anyway.

Benedict is in the business of killing gods, not promoting destruction and chaos. The Companion being unable to get a read off the priestess makes him uneasy, too similar to the Inquisitor in such a regard, and trusting anyone in this city seems wrong anyhow. Playing along on his own he could stomach, but pulling other people along with him, pushing them towards war, that doesn't sit well. He likes to save others, not condemn them. Besides, oddly enough, it feels a little like betraying his 'god'--and doesn't that just tickle the Companion pink considering they both know Benedict doesn't worship anything, much less Itself. Benedict won't live down the teasing from that for a while yet.

Benedict will politely decline. Something simple along the lines of, "Thank you for your generous offer, High Priestess. I am sorry, but I can't renounce my god for another." And if she points out again that they both know the whole 'holy man' persona is a farce, he'll just smile benignly at her and stare into the middle space, almost as if listening to something he can hear only in his head.