Indian religion evolved over the centuries. There is little known of Indian religious beliefs during the Harrapan period. It seems that the Harrapans were very concerned with fertility, as their main god seems to have been a mother-goddess.
Our knowledge of religion during the Aryan period is more comprehensive. The Aryans believed in a series of gods who represented natural phenomena. Gods like Agni, the god of fire, and Indra, the god of war, formed part of the Aryan pantheon. The Aryans believed that the gods enjoyed receiving sacrifices, which were offered up by the priests, known as the Brahmins. The most important part of the religion was how well the rituals were performed.
Religion became sterile and unsatisfying to many, including some of the Brahmins and they retreated to asceticism (severe self-discipline and self denial) to bring new life into the religion. Slowly, the religious standards were reinterpreted. Two central doctrines developed: samsara- the transmigration of souls through their continual rebirth; and karma which determines the type of future life one earns during each successive life.