Yeshayahu Leibowitz noted well, there have been believers and unbelievers in every age.
I posit the true age of atheism started symbolically with the evil French Revolution and de Sade (Al-Ma’arri warrants an investigation). Indeed, the first people to defend atheism in writing appear to be Matthias Knutzen, Kazimierz Łyszczyński, Jean Meslier, and Baron d’Holbach. (Since “atheism” per se is possibly incoherent, I use the word in the colloquial sense of those publically identifying as such.)
Preternaturally-biased Wikipedia has two articles on the history of atheism claiming it’s both quite old, and quite popular since ancient times.
Was Adam Harishon an atheist? Ah, I doubt it. He spoke to Hashem as a prophet. So did his children. Enosh started idolatry.
In Tanach (Scripture) it’s unclear there was even one atheist. Even verses such as אמר נבל בלבו אין אלהים in Tehillim 14 and 53 refer to what stays in the heart (as Chazal say, Iyov sinned by Birkas Hashem in his heart) and might refer to denying Divine Providence alone, etc.
Even pagan societies often recognized One, de-emphasized special “Creator-God”, but focused on deifying their own Henotheistic Mazel\Sar. There are many hundreds of words for Him across numerous cultures, such as the Egyptian “Aten”. As the Psalmist says ממזרח שמש עד מבואו מהלל שם השם, see Menachos 110a “Elaha de’elaha”.
Atheists say every young child is an atheist (and cows are atheists too, presumably!). Well, if we speak of an intellectual discourse, they can’t say too much about “Juice!”, either. And if we speak of the weak “Sensus Divinitatis”, that’s there from the start, including presuppositionally.
Skipping to less ancient times, the very fact “atheist” was an insult, not a proud mantle, shows just how popular it was in generations past. Indeed, this means we cannot know Theodorus the Atheist was one since this title was given him by his enemies (bad example).
Wikipedia includes the names of people who denied the truth of various false deities. But by that ridiculous measure, Avraham Avinu, a literal “iconoclast”, was an atheist, too…! (Not to mention אבנימוס הגרדי.)
Here’s an example:
“Little is known for certain concerning his philosophical views or the nature of his alleged atheism. All that is known for certain on the point is that Diagoras was offended by the worship of the Athenian national gods.”
Then why mention him at all?!
In the fourth century BC, he points to Plato, as the philosopher imagines a believer chastising an atheist: “You and your friends are not the first to have held this view about the gods! There are always those who suffer from this illness, in greater or lesser numbers.”
“Gods” plural. Stop wasting my time!
Mishlei says, 17:28:
גם אויל מחריש חכם יחשב, אטם שפתיו נבון.
Even by the low standards demanded of historians of ancient times (because their work is near-futile), identifications are strictly tentative.
As for “the Epicureans, who were often called “Atheoi” in antiquity, and the atheistic writings of Xenophanes of Colophon“, this may be because they denied Divine Providence, and endorsed hedonism, as we explained elsewhere.
So yes, there were some ancient atheists (Carvakas?), but not too many. Foolish knavery has since increased exponentially.