Christianity is good for its believers - they are better off to believe than to be atheist.

I'd change this one to:

• Christianity is good for most of its believers - they are better off to believe than to be atheist.

~62%

Christianity is good for Christendom - it is a positive force for majority Christian societies, as compared to if those societies were mostly atheist.

~69%

Christianity makes the world a better place, as compared to if all those people were non-believers in any religion.

~58%

Edit: I case it wasn't clear the 70% refers to the disjunction of the above 3.