Three major world religions – Hinduism, Buddhism, and Islam – have been important in India. Hinduism started in India in the Stone Age. Buddhism started in India and spread from India to the rest of Asia. Islam went the other way: it started in West Asia and then came to India.
Nobody knows much about the earliest Indian religion of the Mehrgahr and Harappa people. It was polytheistic – like everyone else in the world at this time, they worshipped a lot of gods. Probably Stone Age and Bronze Age people in India were already worshipping some of the same gods that later Hindus prayed to.
When the Yamnaya came to India from Central Asia about 1500 BC, they brought their Indo-European gods with them. These gods mixed with local gods to produce the Hinduism we know. Hindus wrote down stories about their gods in the Rig Veda and other epic poems. They believed in reincarnation – that people could be reborn into other bodies after they died. They killed animals as gifts to their gods, as animal sacrifices.
But in the 600s BC, animal sacrifice started to go out of style all over Asia. Indian people looked for some other way to get a good rebirth than through sacrifice and the priests. The Bhagavad Gita, written about this time, shows this new interest. So do the teachings of the Buddha in the 500s BC. According to Gautama Buddha, people could get free of the cycle of reincarnation and achieve nirvana by being good people, by learning not to care about their bodies, and by meditating. Buddhism got very popular in India and quickly spread all over East Asia and Iran. But even Buddhists still prayed to the Hindu gods.
About the same time, another Indian religious leader, Mahavira, also encouraged people to try to get out of the cycle of reincarnation. His followers were called Jains. Jains thought that the best way to escape reincarnation was to be good – to be kind to people and animals, and to tell the truth, and not to be greedy.
By the 600s AD, even though Buddhism was still popular in China and Japan, many people in India started to turn back to Hinduism. They still remembered the Buddha, but he became just one of many Hindu gods to them. Jainism kept going, but only a small group of people in India stuck with it. Another small group of people in north-western India were Parsis – Persian Zoroastrians – who moved to India from Iran about 700 AD when most people in Iran converted to Islam.
In the 1100s AD, Indian people also began to convert to Islam as Muslim people from the north conquered India. Islam demanded that people give up worshipping any god but Allah, and so many people did give up their old Hindu gods at this time. But many people, especially in southern India, also stayed Hindus. Most Jains kept on being Jains. And the Parsees in north-western India stayed Zoroastrians, even though the Muslims tried to get them to convert to Islam.