For the people of ancient India,-whatever name it was known then, which was quite vaster in extent than the present Republic nation- the way of life they followed by the teachings and traditions, was Sanaatana Dharma or the eternal way of life. There was no question of calling it as religion, becauseit was just Dharam or Dharma ,the way people are supposed to live right. When people of other Dharam or ways of life different from those practised by the Sanaatana Dharma,those people onlystarted seing, treating the Sanaatana Dharma as different.
These newcomers to the then vast Bhaarat (Bharata Khanda or Bharat subcontinent)called the local people who also followed a life different from the newcomers, as Hindoos .That could have from the words Sindhu of Sindhu river . It was later by the influx of the Europeans that the Sanatana Dharma was treated as a religion.The British government rulers wanted to differentiate the Christianity followers, Islam followers and Sanaatan Dharma followers in their records and that would have started tabulating and treating the Sanatana Dharma as Hinduism or Hindu religion. By then the country had got followers of many other ways and beliefs. So it became in vogue to treat them as religions.
So I am of the view that it was the British rulers who stamped the Sanaatan Dharma as Hindu Religion.
Usually a Religion wil have a starter who originated that. Like Chrisitianity related to Chris, Mohammedans related t Mohammed Budhist related to Budhdha etc. However there is no one specific who can be shown as starter or originator of Sanatana Dharma. There are a lot of rather a stream of rishis, Achaaryas ,Gurus or scholars who made contribution for the explanation, simplification,refinement, improvement and updation of the Sanaatana Dharma tenets and principles to be effectively followed by the people. Such explanations are referred to such and such explanatory or 'Bhaashyakaar' or Rishi .(Example Shakaracharya, Sayana). Similarly the formulas or practical guides followed when needed are referred to the 'Sutrakaar' example Bdhaayana etc.
That is, there is no single person to whom the authority of origination of Sanatana Dharma is attributed to . Hence it is not a religion in the sense of present day Religion. However due to the compelling reasons made by the British rulers and the same copied and followed as system in new independent nation government also Hinduism is finding its place instead of Sanaatana Dharma and the it has come to stay and Sanaatana Dharma is just made to be forgotten.
Thus what is known or twisted as Hinduism in the last two centuries was actually Sanaatana Dharma and everyone in the subcontinent was following the eternal value- based life in the landmass of the subcontinent many hundred years before entry of mass invaders and occupiers who first came in the guise of guests and then became occupants. The tenets and principles of Sanaatana Dharma is applicable to every human being , as they are basic values .Hence Sanaatana Dharma encompasses everyone who lives with basic human values and righteousness. It is important to mention that Sanaatana Dharma has also given space to accommodate those who have different thought options within the ambit of Sanatana Dharma.