According to the great Swami Vivekananda , Hinduism or Sanatana Philosophy has its Roots deep down unto the last man and family of Bharatavarsha. This has been the work of our intellectual ancestors - the Rishis, Gurus, Sannyasins. Sanatan Philosophy was never limited to literature, books and courtyards. The poorer of the poor also followed rituals from the core of heart with utmost division.
Even today after so many years of multiple invasions and by so many cruel Invaders , the people of the land somehow preserve a few things of these Practices. One wpuld be surprised that the indigenous people who were converted earlier in their family also keeps a few Sanatani practices. One may see common Muslim, Christian, Indian with some Hindu practices. This is a thing which keeps other religions on Envy upon Hindustan and still now they spend huge on missionary activities .
The history is filled with example that manytimes the rulers within this subcontinent who were busy fighting amongst , United against the Greek, the Afghani, Turks, Mongols, Mughals , Brits etc . This is not a small achievement when one draws parallel comparison to the great empires of Rome, Greece, Babylonia, Mesopotamia etc.
That famous quote " Unity in diversity" has also helped us then to fight against outsiders.
This question has been pondered by many intellects and Swami Vivekananda had found out the answer as evident from his lectures.
For further research, one can have multiple visits the countryside of India of different states. The Brits were the one close to discover it and that's why they planned to destroy Indian indigenous institutes that helped Sanatana to stay alive . Today we are in the phase of crippling and I bet in future people may Seldom belief about great feats achieved by Ancient rishis.
Don't forget the invasion through funding is still continuing!