Huh, interesting issue. Perhaps the issue comes from the sort of information we find here ... look at “Religious Themes” where it says:
“By the late 1980s he told one of his biographers (Rheta Grimsley Johnson, 1989) that he identified with Secular Humanism.”
Haven’t read the bio, so can’t confirm this. (I also know next to nothing about Schulz).
Thanks for raising the question, though. If anyone knows more, please post away.