Culturally India was a unified entity for millenia. As in, perhaps not having a unified government, but having the same cultural and literary background throughout the region.
Ancient texts like Ramayana and Mahabharata talk about various places and practices across the length and breadth of India, which are remarkably similar and have a unified core.
The greatest temples of Hinduism are not concentrated in one place, but spread over the entirety of the continent. And those are ancient temples. From Saraswati Temple in Kashmir to the Bhagavathi temple in Kanya Kumari.
The idea that India was not a cultural entity was a British invention