I would argue no, not in the modern sense. If I can paint with a broad brush for a moment, modern atheism is not the lack of belief, but rather the belief in rational explanation. To an ancient being without knowledge of the scientific method (and all that we found with it), a belief in something that explained the world would have been perfectly rational. So to be an atheist would require some explanation to understand how they were using the word.

However, there are fairly direct attestations. Socrates was supposedly put to death for something akin to the catholic concept of blasphemy—rejecting the athenian gods and worshiping his own. Now it seems that these charges were unfounded from Plato's account, but it's still clear from the resulting death sentence that public rejection of the beliefs of the surrounding people wasn't tolerated.

On the other hand, both Protagoras and Epicurus argued for a world view that centered around man—Protagoras argued directly against religion (and objective truth in general) and was exiled from Athens for similar charges to Socrates. Aristotle and Plato both recognized him as basically inventing relativism in a serious way, an ascription that has stayed with him today. Epicurus thought the way to direct your life was not to rely on powers outside yourself (for instance, the gods), but to seek happiness and pleasure and minimize pain—i.e. the modern roots of hedonism.

Both philosophers were well-recognized in the classical world, so yes, I think you can argue that secularism and rejection of religious dogma were very well recognized in the ancient world; However, as you can see from Socrates's and Protagoras's examples that it may not have been a popular form, leading to persecution if they feared it might "spread".

Alright, we have deleted the majority of answers in this thread so I will leave all newcomers with this warning. Please note that, in AskHistorians, the onus is on you to provide appropriate evidence for the historical claims you make. Stating that it can be found on Google is not helpful. Mentioning something you may have heard once is not helpful. Guessing based on what you remember from X number of years after the event is not helpful.

Proper comments for this subreddit should be in-depth, comprehensive, and informative while properly situating the answer in its historical context. It should also be backed by appropriate primary and secondary sources, whether or not explicitly mentioned in the comment (i.e. you should always be ready to provide sources if asked). In other words, answers here should be such as an historian of the subject would give.

Thank you.

as /u/goethean mentioned there are atheistic schools of thought in anceint India. Although Samkhya did not believe in dieties, it still held a view of the supernatural. A true atheistic school would be the Carvaka, which held the belief that no gods exists and consciousness ends with death (this implies materialism, and hence fits a modern atheistic thought). Though the Carvaka are very old (around 600BC) I'm not sure if they are the oldest.

Another question, which I don't hold the answer to, is how secretive these atheistic schools of thought were. If one were charged with blasphemy for such thoughts, they would guard them very closely to not be persecuted. This is just speculation, but I think many schools of thought may have been lost due to secrecy.

From the Discourses of Epictetus, Book 1.12 On Contentment, as recorded by Arrian of Nicomedia, c 108 AD.

"With regards to the gods, there are some who say that the divine doesn't even exist, while others say that it does exist, but that it is inactive and indifferent, and exercises no providential care; while a third set of people maintain that it both exists and exercises providential care, but only with regard to important matters relating to the heavens, and in no way to affairs on earth; a fourth set declare that it does take thought for earthly and human affairs, but only in a general way, without showing concern for each particular individual; while a fifth set, to whom both Odysseus and Socrates belonged, say "not a movement of mine escapes you [god]"

Epictetus doesn't really go into what this atheism entails, he is more interested in talking about his interpretations of providence and refuting the claims of more popular competing schools such as the Epicureans, but this passage demonstrates that, while these ideologies might not have attracted huge numbers of followers, they warrant mentioning in a list of possible positions on the existence and purpose of the divine.

The closest I can think of is Lucretius (and hence Epicurus, whose philosophy Lucretius followed). Lucretius was a Roman poet, and his work argues that the entire world functions on its own without any interventions from gods, and that religion is a bad institution that causes unnecessary fear and other problems. So he wasn't exactly saying that the gods don't exist, he was just saying that they have nothing whatsoever to do with the world. He then goes on to give some primitive but surprisingly well-thought out proofs for things like atoms and conservation of energy.

The Romans (and I think the Greeks also, but I know more about Rome) didn't think of their gods the same way we think about ours. There wasn't really a question of whether or not they existed. For them, they could see the lightning, and that was Jupiter. They could see the ocean waves and feel the earthquakes, that was Neptune. The personification and stories about all the gods were, as far as they were concerned, guesses and old stories that may or may not have actually happened.

They couldn't really envision a concept of a different religion, or of people worshipping different gods. When Caesar invaded Gaul and encountered a different culture and religion, he didn't say "oh, these people are wrong about the gods" or "They don't believe in our gods and believe in lies" he said "these weirdos worship the gods really strangely and call them really weird names." They would synthesize other culture's gods with their own all the time- worshipping Isis (from Egypt) was really popular in both Greece and Rome for quite a while, and she existed perfectly well alongside Mars and Ares because they didn't have a concept of there being different religions.

Well, that went kinda off topic and I could probably go on forever, but I hope that was interesting.

"Atheism" and "theism" as categories sit a little less easily in China than elsewhere, but I'm pretty sure Wang Chong would be considered an atheist.

The thing is, he wasn't really arguing against a theistic system, really. He did write a bit about how ridiculous it was to believe in ghosts and spirits, and how things like weather were caused by natural, material processes.

Mainly, his work was dedicated to separating tian (literally “heaven”, but more or less divine forces, all that free-floating energy in the universe) from an individual personality or willpower. It doesn’t reward virtue or punish wickedness, and has no sense organs to see or hear what humans do. Ming, or destiny, exists, but not as an extension of tian. It’s just something we can become aware of after the fact, and by examining specific (material) features… like physical qualities that contribute to health or illness in a person, or political organization that contributes to military success or failure in a state.

He was writing around 100 C.E., but only became really influential much later, during the Qing Dynasty, as a kind of Eastern precursor to the Western materialism that seemed so linked with technology and all that great stuff. Then, once the Communists took the reigns after the Qing, Wang Chong’s materialism (and criticisms of Confucius and other “traditional” Chinese philosophers) was A-OK with the political powers.

I can answer for Ancient Greece, though permit me to speak in generalities for time's sake. The answer is yes, though with two qualifications. Many philosophers in the ancient world advocate forms of materialism. This was very common throughout the Ancient Greek world from Thales to Epicurus. In some cases these accounts seem to actively violate belief in Gods. Now whether the systems are meant to account for Gods (just making them material Gods, as in Epicurus) is not always clear.

While not all philosophers would have been Atheistic in a hard sense (meaning believing in no Gods), many actively mocked the existence of Greek gods. Xenophanes advocated monotheism, Heraclitus outright mocks Homer as a knownothing.

It also seems that natural science was viewed as an Atheistic procedure. In the Apology, Socrates is accused by Meletus of not believing in any gods at all (26e). One reason that Meletus seems to give for this accusation is that "[Socrates] says that the sun’s a stone and the moon earth." (26d).

The other qualification is that in the ancient Greek world, participation in the rites and rituals of the religion was part of what it meant to be a good citizen. Thus, even if you didn't believe in the Greek pantheon, you still went and participated in the traditions. This is where it gets tricky. Freedom of thought was generally quite open in the Greek world, but it stopped when people wouldn't engage in the rites and rituals.

For primary texts and commentaries, see Readings in Ancient Greek Philosophy.