Belief in religion is a normal part of the human experience, even though the beliefs differ and some folks don't have them. These beliefs are generally harmless at best and for some, they help folks have community, a purpose, and all of these other positive things. Sure, some negatives, but that's life.

Normalcy plus lack of the belief causing life problems. Doesn't sound like general sickness to me.

Naturally there are grey areas and exceptions and cults and all this sort of thing, but it doesn't take religion to produce them.

I should at this point mention that I'm atheist.

So you're only delusional if it impairs your ability to function normally within the context of your society. Or if it negatively affect your quality of life, like, say it prevents you from sleeping, makes you overly anxious, etc.

In that sense, I think most common believer of a religion aren't affected in their normal functioning within society, and don't feel negatively affected by their beliefs. But some are, and those could be called out as having mental disorders.