You're know nothing what you talking about , for example i claims that I am Christian lets suppose but i have different religion lets suppose Islam, so how Govt of India is able to my true religious identity, please explain me
Government of India does not know.
Many Christians actually claim that they are Hindus as that is the only way you could avail the government provided reservations (affirmative action) for education & jobs.
This is the reason why Christians in India are under reported by government's population census which is the only time government collects the information about one's religion.