Hinduism is the oldest religion in the world. Prior to that there was no religion. People used to follow a way of life that was called "Sanatan Dharm". Due to spread of Buddhism, Shankaracharya started the propagation of Hinduism and established four "mathas" in the four corners of India and laid stress on the worship of common hindu gods especially Lord Shiva. It was able to curb the spread of Buddhism which was taking people towards atheism. Since then, Hinduism has flourished in India and the subcontinent. Countries of today such as Burma, Thailand, Cambodia, Indonesia, Sri Lanka were dominated by Hinduism. The regions of Burma, Thailand, Cambodia were influenced by Buddhism whereas Indonesia was gradually converted to Islam. Even today, one can see remnants of Hinduism in those countries. Nepal is still a Hindu country and so is majority of India.
In Hinduism there are many gods but in reality even in Hinduism there is only one Supreme God. People consider hinduism to be a polytheistic religion but if one follows the scriptures of Hinduism i.e., one sees that Hinduism is a monotheistic reglion professing the worship of one Supreme God. The mystery is that worship of other gods and deities is more prevalent amongst Hindus and therefore the general impression that goes out is that of polytheism as the most commonly worshipped gods are Lord Shiva, Lord Krishna and Goddess Durga (and many forms of Goddess Durga).