Why is it on here, any mention of religon or race in the context of an OP brings out people who say "I don't see why this is relevant"? In this case, if the DIL is atheist and it's a CHRISTIAN charity, it's relevant because it implies the DIL is not involved in this particular charity. Same as if the OP had mentioned a charity that revolves around a particular ethnicity - mentioning the DIL's ethnicity if it were different would imply the DIL wasn't part of that charity group.
I didn't read anything in the OP to say that DIL was a bad person because she is atheist, or that all atheists are bad people.