Atheism (e.g. The philosophical position that denies the reality of the God of theism and other spiritual beings. Atheists usually affirm as well that the only form of existence is the material universe and that the universe is merely the product of chance or fate) never had a strong following in antiquity but there are ancient records of its existence.
For example, Epicureans were falsely criticized in antiquity with respect to atheism. The atheism of
Epicurus was a false charge. He clearly affirmed the existence of the gods, their immortality and their blessedness (Diogenes Laertius Vit. 10.123.–24).
He did not, however, affirm providence in the sense of divine care and provision for human beings and, in particular, any judicial or punitive role for the gods. Prayer was therefore meaningless to Epicureans, although the contemplation of the gods might serve the purpose of inspiring human beings to some happiness.
In some of these respects, the Epicureans were remarkably like the Jewish sect of the Sadducees. Of the latter Josephus wrote, "The
Sadducees … do away with Fate altogether, and remove God beyond, not merely the commission, but the very sight, of evil.… As for the persistence of the soul after death, penalties in the underworld, and rewards, they will have none of them" (Josephus J.W. 2.8.14 §§164–65).
Plutarch, who, in his Isis and Osiris (377D–378A; 379B-D), had an extended discussion of idols, atheism and superstition.
Flavius Clemens was charged with atheism (Suetonius Domitian 15.1; Dio Cassius Hist. 67.14.1–3).
There were also instances in ancient Israel in which secularism, materialism and disdain of the covenant-ideals squeezed the fear of God out of Israel’s wisdom rendering brief periods of practical atheism (e.g. the effective denial of God in human life) that preceded periods of judgment and defeat.
Oh, and at some point I'm going to get back to Cycel's comment but just haven't gotten it together enough to finish the research phase of my reply. Peace.