Popular or widespread doesn't mean it’s default. Religion is part of culture and that’s something that’s passed down person to person. A blank culture or no culture has no religion. Tabula rasa.

I’m not assuming anything, I’m not pegging the default discontent with death to a particular value, I don’t know what it is and it’s irrelevant. I’m just saying the some religions, with certain concepts of afterlife, like catholicism, would reduce that discontent.