Use 'irreligious' to describe a non-religious stance in a neutral or factual manner. Despite his parents' faith, he grew up irreligious.
Pro Tip 2/3
Respect in Context
'Irreligious' itself isn't rude, but context can make it seem critical of someone's belief. Calling someone irreligious can be seen as judgmental in certain cultures.
Pro Tip 3/3
Not Atheism
'Irreligious' doesn't mean atheist; it implies indifference more than disbelief in deities. He's irreligious; he doesn't go to church or talk about gods.