Because it's a common view that Atheism doesn't apply to Hinduism because Hinduism doesn't require a belief in god, it may be useful to explain how it does.
Atheism is not a worldview. It is a single position on a single matter - the supposed existence of god or gods. People who don't believe that gods exist are atheists. But a religion's not putting much stock in the existence of gods is not enough to make it atheistic. Especially when it's foundations rest on supernatural premises like karma and the afterlife.
An atheist lacks belief in god. But nothing about Atheism prevents one from believing in spirits, magic, cosmic laws, and an impersonal god (like in deism). Atheism can come from many places. Some people reject a particular definition of God while subscribing to another.
This is why a lot of people call themselves "Hindu atheists" or "atheist Hindus". They remove Hinduism from the context of Abrahamic theism and decide that that is enough. They get their material from western atheists, whose primary focus is Islam and Christianity.
It is useful therefore, to see Atheism as an inevitable result of its philosophical base - Materialism. Unlike Atheism, Materialism IS a worldview. It is the view that reality is primarily matter-based. It is a bottom-up set of explanations.
Instead of relying on answers above us, Materialism proposes that complexities are emergent properties of the mundane building blocks of reality. It suggests that we and our reality are not the result of action from above. We come from below.
It suggests that our way of being grew to be what it is after many rounds of trial and error. It's not something that was created as a result of cosmic moral laws or divine agents. Materialism thus does away with the need for karma, gods, the afterlife, or a human-centred world.
I mean, you can still think human beings are important. But Materialism suggests that that is only because you yourself are human. Not because the human animal holds some special position in a top-down order of things.
India had Materialists in 600 BC. So did many other parts of the world. Atheism isn't some kind of modern weapon against the gods. But because religion is pervasive and religious narratives are often comforting, it has something of a secondary place in the discourse.
You will find people saying things like "atheists have no place in a religious dialogue". Nothing could be farther from the truth. If anything, atheists have a special place in the dialogue. They may have an outsider's point of view that religious people lack.
Without atheistic input, religious debates would be limited to the cosmetic level. Which way should the temple face? How old do the girls have to be? Which hand should be used to touch the thingamajig at the Lord's feet?
Without atheistic input, religious debates will be pointless abstract meanderings - fun and convenient for the believers. Religion needs challenges that question its very foundations, not just ones that cause it to change its clothes and move on.