“Unlike monotheism [atheism] did not spread from a single point. In India it antedates the Buddha and the Jina and is found in the Upanishads; in China it was codified by Confucius while a different version was laid down by Lao-tze [...]. While the atheism of the Buddha and the Jina is admitted, frequently Lao-tze's is usually passed over in silence, and the teachings of the Upanishads are glossed over as pantheism.”
Those who do not believe in god(s) are (apparently) insulted in Psalm 14:1, which calls them morally deficient fools. To make this statement, the author must have course been aware of debates and arguments between atheists and theists. The Psalms were compiled between 1050BCE and 600BCE. Although Psalms 14:1 sounds prejudiced, it is followed up by verses disclaiming against everyone else too, calling all people unrighteous and lost.
The philosopher and educator Xenophanes (6th century BCE) is taken by some to be a pantheist (some of his students certainly were), but, he is more directly described as an atheist if you rely on his own words2.
“This universe, which is the same for all, has not been made by any god or man, but it always has been, and will be - an ever-living fire, kindling itself.”
Throughout the 4th century BCE the philosophers of Greece were "profoundly impressed" with the natural world and its operations, and many were of the opinion that the divinities of Olympus were worthless, although at first the uneducated public denounced this "rising... atheism" and punished those who doubted the gods5.
The philosopher Epicurus (341-270BCE) doubted the existence of God because of the presence of such evil and suffering in the world6. Epicurus, Leucippus and Democritus (4th and 5th centuries BCE), were listed by Francis Bacon as examples of atheist philosophers7. Founding atomic theory, they held that the universe was formed from natural causes; atoms coming together by chance, not by the will of a creator. The Epicurean School, a secluded and austere following called The Garden, has been criticized and ridiculed by Christian authors due to their hatred of Epicurean thought:
“The school was much libelled in antiquity and later. [...] In Christian times, Epicureanism was anathema because it taught that man is mortal, that the cosmos is the result of accident, that there is no providential god.”
“In Democritus's conception of the universe, personal gods would seem excluded. [He] works with but three premises: the atoms, their movements, and empty space. From this everything is derived according to strict causality.”
"Atheism in Pagan Antiquity" by Anders Björn Drachmann (1922)9
“[That] the belief in the gods had begun to waver in Athens at the end of the fifth century [BCE], is, in my opinion, conclusive in itself to anybody who is familiar with the more ancient Greek modes of thought. [...] The decisive point is that we posses two quite independent and unambiguous depositions by two fully competent witnesses of the beginning of the fourth century which both treat of the charge of atheism as something which is neither strange nor surprising at their time [in Athens].
"Atheism in Pagan Antiquity" by Anders Björn Drachmann (1922)4
Polybius (born around 200BCE) was a Greek historian. He refers to the educated, the enlightened and the people of the leading political circles and says that for them "it is a matter of course that all this talk about the gods and the underworld is a myth which nobody among the better classes takes seriously"10.
Francis Bacon (London, 1561 - 1626) was a proponent of modern scientific techniques but also an avowed theist who he wrote ignorant rhetoric against atheists. This shows us that atheism was strife in his period, and he bothers himself to attack it. Francis Bacon was a devout scientist and wrote passionately of the need to separate science from religion in order to let science flourish.
The historian Darren Oldridge says that as some people in history were too scared to criticize the powerful church, they would raise some of their doubts while 'possessed'. One such story was compiled by Powell as part of a collection of stories in 1652. A woman called MK thusly voiced her doubts about theism by saying that Satan made her say it:
“There is no God to save thee or punish thee, all things were made by nature, and when thou dyest there is an end of all thy good and bad deeds. Thou talkest of the scripture, and of a God and of a Jesus which thou hast heard of there. See thy simplicity now. How canst thou prove the scriptures to be true? Alas, they were made by man's inventions, there is no hold for thee to take there.”
As the Enlightenment loomed, atheism was visibly popular amongst the educated. Previously, outright atheism was rare in the West. Even long after the Dark Ages, most had learned the hard way to keep any doubts about god or religion to themselves. Censorship laws still made public expression of atheism in lawful in far too many countries - France was particularly stringent14 and skeptics of religion often had to keep quiet, even into The Enlightenment era15. But from the 1700s onwards, history provides a stream of atheist books, politicians, movements and outspoken individuals. There was the group who wrote the first Encyclopedia, led by Denis Diderot (1713-1784). Jean D'Alembert (one of the founders of Positivsm), Baron d'Holbach, the Marquis de Sade (who in 1782 wrote an atheist book), the physician Matthew Turner (another book, in 1782). Some were imprisoned for blasphemy because of their atheism, but from this time onwards largely it has been safe to call yourself an atheist, and the major sciences have since flourished. Not all Enlightenment thinkers gave up on theism - Voltaire in 1764 wrote against atheism, often saying things like "atheists say...", and it is therefore clear that debates between theists and atheists were already common by the Enlightenment16.
“The presence of evil and suffering in the world has even been argued by ... David Hume (1711-76ce) to cast doubt on the existence of God.”
Two Christian Bishops provide testimony to the state of Christianity in the UK in the 18th Century. Bishop Butler in 1736 wrote in his Analogy of Religion that no-one bothered with Christianity, "its fictitious nature being so obvious", and Bishop Watson wrote that "there never was an age [...] in which atheism [is] more generally confessed".
"Britain's first openly atheist MP, Charles Bradlaugh, was elected in 1880"17.
The historian John William Draper in 1881 wrote that "whoever has had an opportunity of becoming acquainted with the mental condition of the intelligent classes in Europe and America, must have perceived that there is a great and rapidly-increasing departure from the public religious faith, and that, while among the more frank this divergence is not concealed, there is a far more extensive and far more dangerous secession, private and unacknowledged"18. The Victorian Era (1839-1901) as noted by Steve Bruce below, was the time was atheism has been at a height of prominence. In much of Europe, atheism is more popular than theism.
"The Varieties of Religious Experience" by William James (1902)19 is the text of lectures from 1900-1901. It clearly demonstrates that the author, 120 years ago, lived in an era where atheism and irreligiosity was strife, particularly in academic circles, and also that there are large numbers of secular-living people, not just in the UK where the lectures were delivered but also in his home country, the USA.
The sociologist of religion Steve Bruce states "it should be no surprise that, though there are more avowed atheism than there were twenty years ago, they remain rare. Self-conscious atheism and agnosticism are features of religious cultures and were at their height in the Victorian era. They are postures adopted in a world where people are keenly interested in religion"20.