I would argue that "morality" is a requirement for mental stability.
If you don't have morals or a code of ethics, you would be okay with stealing, murder, rape, etc. which is not mentally stable.
So if morality or a code of ethics is a requirement for mental stability, what better code or rules that the bible? The bible provides a great list of moral codes and allegories and stories.
Why do you think that someone who bases their morals or ethics on thousand year old texts is a mentally unstable?