Question of the author is whether he can use the expression "Indian" for purpose of religion? Can he adopt if his religion is "Indian"? If yes, then which enactment helps him? If no, then which enactment prevents him?
Religion is a freedom under Part III of the Constitution. Preventing the use of expression "Indian" for purpose of religion can only be done as per legal procedure established by law(due process).
To establish legal procedure by law, central or state legislature needs legislative powers to be delegated to it from the Constitution. For adoption the powers have been enumerated in entry number 5 in concurrent list.
Is central or state legislature getting legislative powers to prevent using the expression "Indian" for purpose of religion in Article 25 or anywhere in Part III or anywhere in the Constitution? Yes or no. It appears the answer is no.
If answer is yes, I would like to read the enactments of competent legislature that prevents the using of expression "Indian" for purpose of religion and prevent adoption for such persons. Religion and adoption are a matter of human rights of both adopter and adoptee. Human rights are part and parcel of Part III of the Constitution.