Where, in any suit or proceeding, it is necessary for any Court under this Act to decide any question regarding succession, inheritance, marriage or caste, or any religious usage or institution,
         (a) the Muhammadan law in cases where the parties are Muhammadans, and the Hindu law in cases where the parties are Hindus, or
         (b) any custom (if such there be) having the force of law and governing the parties or property concerned,
         shall form the rule of decision, unless such law or custom has, by legislative enactment, been altered or abolished.
         (c) In cases where no specific rule exists, the Court shall act according to justice, equity and good conscience.