After the collapse of the Mughal Empire about 1700 AD, the power of Hinduism increased as the Maratha Empire took over most of India. But British colonizers gradually took over more and more of India: first the north, then the south. Wherever the British went, Christian missionaries went too. They tried to get Indian people to become Christians.
In the 1700s Protestant missionaries joined the earlier Jesuit missionaries. A few Indians did become Christians, but almost everybody remained Muslims (in the north) or Hindus (in the south, in the powerful Maratha Empire). The British picked out Parsees and Sikhs to be their allies and get special favors. Parsees and Sikhs became richer and more powerful, but other people began to hate them for their privilege.
When the people of India forced the British to leave India in the 1940s, the newly independent country decided to divide the Muslims from the Hindus. The new government moved the Muslims north into the new country of Pakistan (part of which later split off as Bangladesh), while the Hindus moved to the new country of India further south. Ten million people had to move, losing their jobs and houses. A million people died in the riots and fighting that went with this religious separation, the Partition.
Today, most of the people in India are Hindus, while most of the people in Pakistan and Bangladesh are Muslims. They still spend a lot of time fighting about it.