India is a country with "religious" personal (civil) laws. Its Constitution, adopted in 1947, when the so-called "Hindu Right" was a tiny political minority, has a directive principle, which states that the State will strive to have a Uniform Civil Code, but apart from the so-called "Hindu Right", nobody wants to get there.
So, e.g., to govern marriage, divorce, inheritance, etc., there is a Muslim Personal Law, a Christian Personal Law, a Parsi Personal Law and a Hindu Personal Law. The question always arises, to who does the Hindu Personal Law apply?
The Constitution does not really go into this. However, specific legislation does, so e.g.,
to borrow from Koenraad Elst:
The Hindu Marriage Act of 1955 goes in greater detail to define this "legal Hindu", by stipulating in Section 2 that the Act applies:
(a) to any person who is a Hindu by religion in any of its forms and developments, including a Virashaiva, a Lingayat or a follower of the Brahmo, Prarthana or Arya Samaj,
(b) to any person who is a Buddhist, Jain or Sikh by religion, and
(c) to any other person domiciled in the territories to which this Act extends who is not a Muslim, Christian, Parsi or Jew by religion
This is a legal definition. How it should be related to cultural realities is yet to be determined.
Why is it possible to legally determine who a Muslim, Christian, Parsi or Jew is? Because each of these has a definite creed, a definite Holy Book, definite Prophets and so on.
In fact, in "secular" India, the government can interfere with Hindu institutions, such as Hindu temples or schools; but are constitutionally barred from interfering with non-Hindu institutions. So back iwhen the communist government of West Bengal was giving the Ramakrishna Mission a hard time, the Ramakrishna Mission moved the courts that it is not a Hindu institution.
The Supreme Court in its ruling, way back in 1995 stated:
When we think of the Hindu religion, we find it difficult, if not
impossible, to define Hindu religion or even adequately
describe it. Unlike other religions in the world, the Hindu religion
does not claim any one prophet; it does not worship any one
God; it does not subscribe to any one dogma; it does not believe in
any one philosophic concept; it does not follow any one set of
religious rites or performances; in fact, it does not appear to
satisfy the narrow traditional features of any religion of creed. It
may broadly be described as a way of life and nothing more. Confronted by this difficulty, Dr. Radhakrishnan realized that `
to many Hinduism seems to be a name without any
content. Is it a museum of beliefs, a medley or rites, or a mere map, a
geographical expression [The Hindu View of Life by Dr.
Radhakrishnan, p.11]]
(Dr. Radhakrishnan was the first Vice President and the second President of independent India.)
and therefore, with this reasoning
In the result we dismiss these appeals, however by setting aside the
holding of the learned single Judge in the Writ Petition
that Ramakrishna religion being a religion distinct and separate from
Hindu Religion was a minority in West Bengal based on
religion, entitled to protection under Article 30 (1) of the
constitution of India as upheld by the Division Bench of the High
Court in its judgment deciding the appeals before it and also by
setting aside the holding of the Division Bench of the High
Court that Ramakrishna Mission as a religious denomination was
entitled to establish and maintain institutions of general
education under Article 26 (a) of the Constitution of India as those
established and maintained for a charitable purpose.
PS: one might, based on behavior observed from a distant ship, think that sharks and dolphins are the same, e.g. when they swim at the surface with their dorsal fin sticking above the surface. Having analogous behavior does not make them the same, one has to look deeper into the structure. Casting the Indic traditions as religion is akin to someone who only knows sharks, insisting that dolphins are sharks, and throws into doubt whether the someone understands even sharks.