As regards your study of the Hindu religion: The origins of this and many other religions that abound in India are not quite known to us, and even the Orientalists and the students of religion are not in complete accord about the results of their investigations in that field. The Bahá’í writings also do not refer specifically to any of these forms of religion current in India. So, the Guardian feels it impossible to give you any definite and detailed information on that subject. He would urge you, however, to carry on your studies in that field, although its immensity is wellnigh bewildering, with the view of bringing the Message to the Hindus. The task of converting this section of the Indian population is a most vital obligation, although the Guardian is fully aware of the many difficulties that it presents. Nevertheless the friends should do their best to make as many converts among the Hindus as they possibly can.
(Shoghi Effendi, The Compilation of Compilations vol. I, p. 19)
With reference to your question concerning the Sabaean and Hindu religions: there is nothing in the Teachings that could help us in ascertaining which one of these two Faiths is older. Neither history seems to be able to provide a definite answer to this question. The records concerning the origin of these religions are not sufficiently detailed and reliable to offer any conclusive evidence on this point.
(Shoghi Effendi, The Compilation of Compilations vol. I, p. 20)