I think medically, most mental disorders are characterised by a kind of normal functioning and quality of life metric.
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.
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.