We often use the term Hindu Dharma, Dharma meaning way of life. But this does not exactly transforms to western concept of a religion where the people have similar practises and worship same God.
What is the unifying concept in Hinduism (God, food habits, cultural practises)? Actually nothing except that, all the communities have their origins in India. But then Jainism, Buddhism, Sikhism.. all are originated in India. Then, why these communities are not Hindus? For example, a Brahmin who worships fire and vegetarian by food habit, a villager in Tamilnad, who can drink animal blood which he offers to his village deity, a Jain who worships Arhan and a total vegetarian... all are unique in their ways. Why first two became Hindus and the last one was grouped as a separate religion?
There is one difference. The last community has a synthesised faith. It has a founder (Mahavir). So are Buddhism and Sikhisms. They have a founder and they also have a guiding book. In other words they are equivalent to our western counterparts of Abrahamic religions, which also have Prophets, guiding books.
But what about the first two categories (A Brahmin and a tribal in a village). Their history may be much older than these religions, their practises may be as unique as others. But, they don't have a phophet (as they don't even know when their faith started) or a book. So they can't be classified as separate religions. Acharyas (prophets) are present in Brahminism but, Brahminism was existing for centuries before they arrived. Though Vedas are 3000 years old, nobody says it is an ultimate book. So Brahminism is not a religion. With 3 Acharyas and 3000 year old Vedas, if Brahminism did not get a chance to be called a religion, what about others who did not even have that? So, none of them could make it to that elite group religion. Only way was to keep the communities out, which are already branded as religions. Rest, bundle them together and package with the label "Hindu".
Just a thought, what happens if this roof joining all of them, the so called Hinduism is taken off and each are declared as a religion? Nothing. Actually these communities never mingled with each other, anyway. All of them have preserved and maintained their uniqueness (cultural practises), have got married within their own communities even otherwise. So branding them together by a name or separating them out, both does not make any practical difference. Then why was a brand name required for us (Indians)?
It was not a requirement for us, that is why we did not bother. In fact, when Krishna speaks about "Swa Dharma" (one's Dharma) in Gita and if Dharma actually means religion, then for him each caste was a religion. The term "Hindu" was unknown to Acharya Shankara (788 - 820) (who is supposed to have defended Hinduism from turning itself to Buddhism). But it became a requirement for the people outside India. They wanted a frame of reference to describe Indian's faith. They imagined a hypothetical religion lying outside river Hindu, clubbed all this bunch of faiths and called it Hinduism. Alberuni's “Tahqiq-i-Hind” (973-1048 ) discusses about religion(/s) in practised in region after river Sindhu (Indus). Further later Varnaashrama became part of obscure religion Hinduism (where these communities were called castes), just the way Bharata Varsha became India in outsiders tongue. We went with the flow and admitted both these terminologies.
In this regard, Hinduism is actually multicultural society wrongly understood as a religion by outsiders. When its contemporaries (Rome had collection of tribes worshipping different Gods before the ascent of Abrahamic religions. Pagan's fight with Christianity lasted only for few centuries) faded long ago, while the westerns countries are ready to accept muliculturalism has failed in just few decades, here we have the longest surviving multicultural society. That itself makes the study of Hinduism fascinating. (Whenever I use the term Hinduism further in my posts, I just imply the collection of these communities, not as any religion).