Hindu is a word coined much later when Islamic invaders attacked and occupied India (erstwhile Bharatvarsha). Initially existed Dharma which was eternal, existed from times immemorial. Even before a single religion advent on mother earth existed Dharma (righteousness)! Even before any religion existed in world, mankind conducted itself through eternal Dharma.
When highly evolved Sages and Rishis invoked direct messages from God that were finally compiled into Vedas, the collective thinking, all the doctrines collectively were known as Hinduism. As Dharma is always eternal and all the doctrines existed from times immemorial, Hinduism is also called Sanatana Dharma, eternal Dharma, the world Sanatana meaning eternal.
The first words of God to appear on earth were Vedas popularly called Hinduism revelations. Hinduism, Sanatana Dharma is one tradition that exists from the beginning of life on mother earth.
This analogy can be understood as follows:
Initially came, existed tree of Dharma which was eternal hence Sanatana Dharma. As the tree of Dharma grew manifest Lord Krishna, the first avatar that advent on mother earth. As Lord Krishna himself was Dharmaraja (embodiment of Dharma), no religion grew after him.
As the tree of Dharma grew further it developed its first branch and advent Mahavira, 24th Tirthankara of Jainism. Another branch and advent Gautama Buddha! With next branch advent Jesus Christ and another branch into Prophet Mohammed! As the tree of Dharma flourished many religions world over came into existence.
The tree of Dharma is still evolving but one absolute fact of life can never be disputed that Hinduism, Sanatana Dharma is the oldest and the best religion of the world.