The earliest recordings of atheism is from the Greeks, more specifically Lucretius, Epicurus and Democritus: Lucretius 99 BC - 55 BC Epicurus 341 BC – 270 BC Democritus 460 BC - 370 BC

Where? It became mainstream in China and the USSR when the communists took over if I remember right. It is still not "mainstream" in the USA or most Catholic or Orthodox countries. Perhaps I have misunderstood what you mean by "mainstream."

First of all atheism is not a belief system and as far as I know it was never mainstream. Is the question a trick question?

Atheism is not a belief system. If you look up belief in the dictionary you can make an argument out of that because the word belief is sometimes a broad term but in this context it is used is wrong. And besides atheism is not a system. As far as it being mainstream , I wish it were to many people stuck in dogma

The first people to doubt the existance of any God or creater were back in the time of the Greeks. Some intellelectual philosphers started to see the world could be explained without God. By the time of the Romans a very small number of the public were atheist, Roman grave stones have been found clearly stating that the occupant did not belive in God. However, atheism didn't become common until the 19th and 20th century. After the idea that the Earth went around the Sun, and that some rocks on Earth were billions of years old and Darwin's Theory of Evolution athiesm became increasingly common, and recognised. After the World Wars the number of religious people started to plumit in many countries. Today the UK sees an average of 1 million people in it's churches each week out of a population of 60 million, and that's set to half within 10 years; France has declared itself a secular state; other countires like China actively discourage religion; and most developed nations have a large proportion of atheists, often more than 50%.

I suspect that atheism predates theism. Just like nudity predates wearing clothing.

The thought that there is nothing to believe in has been around for some time. In the USA Madeline Murry O'Hair made it better known and it came to be known more in the homes of American. Madalyn Murray O'Hair (April 13, 1919 – 1995) was an American atheist who founded American Atheists, and campaigned for the separation of church and state. She was murdered at age 76. Biography Madalyn Mays was born in Beechview, Pennsylvania in 1919. As an infant she was baptized into the Presbyterian church. She married John Henry Roths in 1941. They separated when they both enlisted, he in the United States Marine Corps, she in the Women’s Army Auxiliary Corps. In 1945, while posted to a cryptography position in Italy, she began an affair with William J. Murray Jr., and she gave birth to a boy (William). Murray was a married Roman Catholic, and he refused to divorce his wife. Nevertheless, Madalyn Mays divorced Roths and began calling herself Madalyn Murray. O'Hair completed a BA from Ashland College. In 1952, she completed a law degree from South Texas College of Law, but she never practiced law. On November 16, 1954, she gave birth to another son (Jon Garth Murray) by a different father. Murray attended meetings of the Socialist Workers Party in 1957. In 1959 she applied for Soviet citizenship. The following year, having gotten no response, she and her two children traveled to Europe with the intention of defecting to the Soviet embassy in Paris. The Soviets refused them entry. Madalyn and her sons returned to Baltimore in 1960.[1] In 1960, Murray filed a lawsuit (Murray v. Curlett) against the Baltimore, Maryland School District in which she claimed it was unconstitutional for her son William to participate in Bible readings at Baltimore public schools. She further went on to claim that her son's refusal to partake in the Bible readings had made him the victim of violence from other classmates, violence that she claimed was overlooked by administrators. (Her son William later publicly averred that her claims of him being a victim of violence were fraudulent; see below.) In 1963, this suit (amalgamated with the similar Abington School District v. Schempp) reached the United States Supreme Court which voted 8-1 in her favor, effectively banning "coercive" public prayer and Bible-reading at public schools in the United States. Madalyn Murray became so controversial in her opinions that, in 1964, Life magazine referred to her as "the most hated woman in America." Before Life, Robert Anton Wilson had written an article with the same title for Fact Magazine. It was the article in Fact Magazine that had prompted Life to run their article. [edit] The founding of American Atheists and later Following the Supreme Court decision, she founded American Atheists, "a nationwide movement which defends the civil rights of non-believers, works for the separation of church and state, and addresses issues of First Amendment public policy." She acted as its first CEO before later handing that office on to her son Jon Garth. In 1965, Madalyn married Richard O'Hair. Throughout the 1970s, she publicly debated religious leaders on a variety of issues and also produced an atheist radio program in which she criticized religion and theism. She filed lawsuits on many issues over which she felt there was a collusion of church and state in violation of the Constitution. At this point, Richard O'Hair disappeared from her life. [citation needed] In 1980, her son William converted to Christianity and was born again at a Baptist Church in Dallas, Texas, where he took up work as a preacher. This led to a permanent estrangement between mother and son. As she put it, "One could call this a postnatal abortion on the part of a mother, I guess; I repudiate him entirely and completely for now and all times...He is beyond human forgiveness." [2] In sermons, William accused his mother of using him as a tool in her crusade, claiming she had lied about her reasons for filing the lawsuit against Maryland, and that he had never been the victim of any kind of violence at the hands of his Christian classmates. He said that the true reason for his mother filing the suit was her deep personal hatred for followers of Christianity. He said her zeal against Christianity was so great that it had taken over her life and rendered her incapable of seeing other people (himself included) as anything but either enemies or people who agreed with her every ideal. O'Hair was also critical of many other atheists, even expelling a few people from her organization. In a 1982 address, she criticized a wide variety of atheists as being unacceptable. From Wikipedia, the free encyclopedia

I would wonder whether atheism has ever been "mainstream". Since the central tenet of atheism is "I do not believe", atheists rarely group together unless under threat. Since atheists tend to practice their unbelief separately and privately, you will never know how many of them there are.

Well, ya see, it's not a belief "system," and that's kind of the point here. I think that mostly Atheists just don't feel that they need a belief "system." If anything, my belief "system" involves people taking responsibility for their own actions.

It was a short while before God existed, and its been a short while since Gods been dead.

The oldest profession in the world is Shaman. The people who said "hang on a minute, he's getting all our food and pretty things because he says there's a Deity who will punish us unless we give him food and pretty things, but he can't actually prove any of it" were Atheists. So I'd say that Atheism became mainstream about the same time that some human got the idea to make up some religious stories in exchange for never having to work physical labor another day in his life.

Buddhism is an atheistic belief system and it's been around since the 5th Century BC.

Atheism is not a bleief system. It is a lack of belief in a theistic system. The common phrase is "if atheism in a religion/belief system then bald is a hair colour!" Atheism goes back millenia. But in modern times in the west at least I think its intellectual roots can be traced to Deists like Hume and particularly Tom Paine.

"I'm not an athiest. How can you not believe in something that doesn't exist? That's way too convoluted for me.” -A. Whitney Brown

Atheism is the natural state upon birth.

People have pretended to be atheists for thousands of years. Psalm 14:1 The fool hath said in his heart, There is no God. They are corrupt, they have done abominable works, there is none that doeth good.

It's not a belief system. There have always been people who haven't bought into the claims others make about gods, but they were usually killed. Nowadays not so many atheists are tortured and killed by the faithful, so it's a bit safer to admit that we don't buy it.

It's not a belief system. And I'm not sure about the mainstream thing.

Never. It isn't a system. It is about DISbelief. Last I checked we were still in the minority.

It never has been. Mainly as "atheism" is just an opinion with scientific pretensions. It has no credence in facts or logic. Logically you can only be a theist or an agnostic.

* It is ironic that in the Roman Empire, one of the accusations brought against the early Christians was that they were atheists. Dr. Augustus Neander writes:

it's not a belief system. And with less than 7% of the population claiming to be atheist is hardly mainstream either.

9-5-2017 In the 19th century Americans started moving west and three institutions went with them. There was vaudeville, traveling entertainment. There was lyceum, traveling education and culture. And there was the itinerant preacher, offering a new style of preaching called "hell fire and brim stone". It was very entertaining, only loosely based on scripture, and pastors didn't even try to compete. Instead they switched to preaching public morality and philosophy. Eventually an entire generation grew up not knowing the first thing about the religion they claimed to believe. That is why most Christian churches don't teach doctrines, and most members don't know what they are supposed to believe. American atheism was simply people smart enough to notice that Christians no longer knew what they were talking about, although it took a while to develop. The public aspect dates from "Gone With The Wind" when Hollywood censorship was finally exposed as a toothless guardian of morality, and with Hugh Hefner's leadership, the blame for that was placed squarely on the Christian church.

Atheism has been around since the Genesis account of Noah. According to Scripture, atheists were described as godless people, which means ungodly. 2 Peter 2:5 "And spared not the old world, but saved Noah the eighth person, a preacher of righteousness, bringing in the flood upon the world of the ungodly;" In today's generation, atheism went mainstream in 2006 by a journalist named Gary Wolf who described the ungodly promotions and articles atheists were publishing. After 2006, atheism was receiving more publicity, as millions of people were getting attracted to social media. What atheists want you to accept is that atheism is not a belief system. In fact, it is a belief system and I'll prove it. (When they push their ungodly beliefs, they are trying to convince you to believe that there is no god). If they really did not believe in a god, they wouldn't spend their time and energy on religious categories or subscribing to Religion.

when man FIRST decided to follow his EGO

Freedom of choice, looks like democraps are deep into that and most of their thinking!