For J4J and the like, I usually focus on the fraud -- rather than being up front about their Christianity they had behind a facade with the intent of duping people. Not only is that deceptive and insulting, but doesn't it run counter to Christian values? Honesty is still supposed to be a core value, right? And if you're so embarrassed by the religion you're promoting that you try to *hide* it, well, what does that say?

For Christianity in general, I usually talk about how a core tenet is our destruction. Originally physical destruction (pogroms etc) and there's still some of that out there, but also ideological. A core tenet of Christianity is "Jews were wrong about God and we are here to replace them with version 2.0 which is obviously better".