Who are Indians are they not the same people who decended down from the people who happend to live in the valley of INDUS ? And as such called Hindus? The religion of Indians may be different but is it not a fact that an Indian is a born Hindu?
Hinduism is the oldest religion and most people in India were born as Hindus. Ofcourse there have been waves of migrations from across the HindKush and the Mongol North but they have all jelled in the melting pot called INDIA
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All citizens of India are Indians,irrespective of their religons. Hiduism is a culture of followings , the people who lived here in India are influenced by this culture, that's why the Indian muslims are also called Hindu muslims at mecca. Most of the hindus are following the Santan Dharm.