Is believing in a deity a mental illness?

It is more complicanted than that but it does show us athe person has not had mature thoughts or has chosen not to thing more maturely. When I found out that Sarah Palin has 3 out of her 4 grandparents who are Jewish I wondered if she was told when she was little that she could see Russia from her back yard. Of course, I don't know if the family came from Russia but now it makes sense to me that she has been told this. Many people don't even believe she said this & accuse SNL of making it up but it is on youtube.com for those who didn't see her when she originally said this. So why would a middle aged women belief this, I don't know. It is officially called a mental illness but might have another name.