Art. 25 of the Indian Constitution says (in part)
- Subject to public order, morality and health and to the other
provisions of this Part, all persons are equally entitled to freedom
of conscience and the right freely to profess, practice and propagate
- Nothing in this article shall affect the operation of any existing law or prevent the State from making any law
restricting any economic, financial, political or other secular
activity which may be associated with religious practice;
providing for social welfare and reform or the throwing open of Hindu
religious institutions of a public character to all classes and
sections of Hindus.
The preamble does also declare that India is a sovereign socialist secular democratic republic, but this is not a clearly-enforceable article. There is no legal requirement that government action be entirely devoid of consideration for religion. What the law says is that the government must respect the rights of individuals to practice their own religion, but that does not preclude favoring a particular religion, e.g. using Hindu texts in official contexts. So far, the courts have not rules that government action must be entirely devoid of religious reference (for example the various Hindu family laws vs. Muslim family laws are not unconstitutional).