What does Hinduism have to say about atheism?

Sanathana Dharma, aka Hinduism, is essentially Atheistic in that it doesn't accept the Concept of any Supreme Being creating the World, nor in there being any Heaven nor Hell. It postulates that we are truly the Eternal Soul - Atman and are at once One and Several with the Collective Universal Consciousness - the Parama Atman. The "na'astik" as you called it.

The polytheistic Hinduism of today is not the original Sanathana Dharma that was the Gnyana Marga or the Path of Intellectualism and Science, but, the Bhakthi Marga or the Path of Faith, that is borrowed from the Greek Pantheon and Judaism and Christianity.

You can see that they Indianized Zeus [Indra] and Poseidon [Varuna] etc and Adam and Eve [Manu and Shataroopa] and the tale of the Great Deluge [Matsya Avatar] and the Trinity etc from Christianity.

Sanathana Dharma is a tolerant Philosophy that allowed the most diametrically opposite line of thinking - Theism and let the people choose their own Personal Gods [ichcha devas].

The basic rationale behind it is that they believe that, in the End, everyone, including the Bhakthi Margis and the non-Hindus, will learn the Final Truth, so, they do not oppose them.

As you must know, the most fundamental tenet of Sanathana Dharma is "ahimsa paramo dharma" - Non-violence, in thought or deed, is the most desirable Path.

Now that I have laid the foundations for my answer, let me answer your specific question.

Sanathana Dharma does not have anything to say about any Concept that is foreign to it's ambit. All it says is "Do not hurt them in thought or deed because whether they like it or not, they will get the Truth in the End and gain the Final Knowledge."

Incidentally, there's no such thing as the "True Form of Atheism" Atheism can be Agnostic Atheism etc etc.

Nastik Hinduism is closer to Agnostic Atheism.

There is nothing to say about atheism. I believe in multiple manifestations of Brahman a.k.a God and atheists believe in zero deities.

What does Hinduism have to say about atheism?
When I say atheism I do not mean "nastik", but atheism in its true form as it is meant in English

~~~ Hindus are busy, in the moment, dealing with their own 'problems', finding the unconditional Love that is Enlightenment!
They are not hypocrites running around persecuting others who believe differently.
Unconditional Love is just that, unconditional!
Atheists, like all, are the Beloved!
For that, you need a Xtian!

This is in response to Anonymous

You have written here about Jainism, not Hinduism. Though you may argue that Jainism evolved from Hinduism just like Buddhism, but I was searching for the answer from Hinduism and not any other corollary of it.

Secondly, I would have to disagree with you about what Hinduism has taken from Christianity. You wrote that Hinduism had 'indianized' Greek and Christian gods. But this cannot be true and is , thus, false. This is because, Hinduism predates any religion in the world. Christianity was only born about 2000 years ago whereas Hinduism was born much much before that.

Rig Veda, which is the oldest known surviving literary work, dates back to 1900 bce. Thus, predating any religion.

I'm still searching for what Hinduism says about atheists

Hinduism believes in many gods, deity's, and it is a practiced religion, they will say the same of atheists as other religious do, unbelievers.