Nullifidian

Near-synonyms: infidel, apostate, heretic (either coordinate or synonymous depending on the beliefs of the speaker or writer)

[...] Celia was no longer the eternal cherub, but a thorn in her spirit, a pink-and-white nullifidian, worse than any discouraging presence in the 'Pilgrim's Progress.'

A man may be content to remain a nullifidian; women cannot rest at that stage. They demand the spiritual significance of everything.

Near-synonyms: atheistic, agnostic