The most popular religion in Ancient India was Hinduism. It was an important part of the country's culture. It was considered as a polytheistic and henotheistic religion with dedication to a pantheon of gods.
That would be Hinduism. Hindus believe that you are reincarnated into something else after you die. They also have a concept you've probably heard of called Karma, which is basically how they weigh your good deeds against your bad ones.