I once heard that the Chofetz Chaim has a letter in which he writes that even someone who is totally non-religious, if they are killed because they are a Jew, they have a portion in the world to come as if they were religious. Since they identifed as a Jew (which is why they were killed), they're considered part of the fold.
Where can I find this letter? I skim read Michtavei Chofetz Chaim and didn't see anything remotely connected to this topic. But I definitely could have missed it.