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Irreligious
Impersonal Tone
Use 'irreligious' to describe a non-religious stance in a neutral or factual manner. 
Respect in Context
'Irreligious' itself isn't rude, but context can make it seem critical of someone's belief. 
Not Atheism
'Irreligious' doesn't mean atheist; it implies indifference more than disbelief in deities. 