The documented spiritual history of India begins with the Rig Veda, conservatively dated to around 1500 BCE. From this root developed the subcontinent’s orthodox philosophical systems, such as Vedanta and Yoga. Buddhism, Jainism and other unorthodox systems developed in opposition to Vedic orthodoxy but retained some of its insights.
During the medieval and modern periods, India reacted against but also absorbed some aspects of the religious traditions of the cultures that entered from the north and west, in particular Islam and Christianity.
My book Indian Religions (NYU Press, Hurst Publishers, Permanent Black, 2002) explores the rich spiritual heritage of South Asia.