What could have helped the Indians to rally in the face of the onslaught of the Europeans? Could it be Islam or Hinduism?

Although the colonialists brought a lot of grief to the Indian people, they also helped to rally the Indians in the face of a common threat. In addition, the British built factories, railways and schools. Young people from wealthy families went to study abroad, bringing new knowledge and ideas from there.
The absence of a strong centralized state in India had a detrimental effect on the fate of this ancient country. At first, it became the target of attacks by warlike neighbors, and then the treasures of India attracted the attention of European powers.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.