It was promised that my previous argument would be better than D’Souza’s in his upcoming debate. I merely equaled his performance-to-come. Apologies.
The irrefutable argument that Christianity has been “good for Mankind” is two questions: 1) who gets to define Christianity, us…or Christ? Followed inevitably by, 2) when is “love thy neighbor as thyself” bad for Mankind?
Please note: the argument addresses Christianity’s results, not its veracity. And this—come to think of it—doesn’t work for me at all. If something isn’t true then I don’t care how much misery and suffering its existence prevents. It’s still not true.
Walter Kovacs couldn’t bring himself to lie but knew what would happen when he spoke the truth. So he goaded Dr. Manhattan into killing him. Which means I’m less reasonable than Rorschach…
World's Only Rational Man 
I’m trying to follow, and agreeing with a lot, but it’s like you’re referring to arguments made in a post I haven’t read yet.
I also find a lot of these kind of debates irksome and I don’t even keep up with them any more. Like you allude (I think), the starting premises guarantee that what is being argued about is confusing and superficial. And everyone’s bristling with agendas, including me if I’m stupid enough to get involved in them. But I actually like talking about religion in a serious way with atheitsts and skeptics or other faith or other denominational people.