While it's true that religiosity has declined globally, residents of the United States may be biased by the outsized decline, which has outpaced most countries. The political scientist Ronald F. Inglehart (who sadly passed away this year) studied this and many other questions for decades in the World Values Survey.
In 1989 when the survey was first posed in, 79% of Americas reported that religion was either "Very important" or "Rather important" in their lives. This same figure is down to 61% today. So, very roughly speaking, about 18% of Americans have become secularized in the past 30 years, so it makes sense that Americans would regard this as a massive, ground-swelling, global change.^(1)
But the effect does not hold universally. In Egypt, for instance, the modern survey and the first survey year (1999) found identical results for "religion is very/rather important"—100%. So a resident in one of these countries might be a little flummoxed by the premise that religion has declined.
In Ukraine for instance and other former Soviet states, we have actually seen a slight increase in religiosity recently, likely owing the ongoing erasure of strong atheist values left over from communism.^(2) 1989 really marks the re-emergence of religiosity in Ukraine. It was in this year that essentially all religious leaders were released from Siberian captivity, and Gorbachev revealed that he had been baptized as a child, and publicly visited with the Pope.^(3)
But enough with specific examples, you're asking about the general trend, and yes, religiosity has declined globally.^(4) What accounts for this? In Inglehart's telling, it is the emergence of sexual liberation, contraception, and gender equality that has led people everywhere to question the rigors of faith which demand strict gender roles and high fertility.
Getting back to your original question re: "has this happened before?" we can find a few notable examples surrounding times of massive upheaval and, in particular, progressive political revolution. For instance, revolutionary France saw the virtual collapse of christianity, with Diderot famously saying "Man will never be free until the last king is strangled with the entrails of the last priest."^(5)
Perhaps most famously, the Russian revolution of 1917 ultimately resulted in a state-sponsored doctrine of scientific atheism, though it's unclear to what extent Soviet citizens took this world view into their heart vs. performing it to satisfy state mandates.^(6) It is also unclear which data to trust from this period.
Finally, I would caution against looking at a local trend in the recent history of religiosity and imagining this to be the permanent and final state of religious devotion. We have seen historically that religiosity ebbs and flows in keeping with the values of the day, and we have every reason to believe that will continue.
Citations
While waiting for a response to this specific question, here are some previous links that might be helpful:
Was everyone religious in the old days, like Medieval Times, or were there irreligious people? by u/mikedash
The AskHistorians podcast episode 151: "Medieval Atheism" with Keagan Brewer
I’m going to look at this through the perception of declining religiosity rather than a modern conception of atheism to avoid prescribing beliefs and to avoid the contemporary messiness of spiritual vs religious which could challenge the premise of the question.
Historians debate how religious decline and perceived religious decline often. For example, in seventeenth-century Massachusetts, the ministry feared a decline in Puritan religiosity. My interpretation is that ministers saw, believed, and acted on this declension, but there was not actually a decline in the overall population. Other historians may disagree, and there are reasonable arguments for it, but in my view, for all the cultural shifts occurring at the turn of the 17th and 18th centuries, the clergy exaggerate and act according to this perception.
Puritans deeply believed in providentialism- the idea that God would punish individuals and communities for moral and religious failings. If communities tolerated people who dissented and challenged the Puritan clergy, it showed a fault and some form of divine judgment would occur. This could be crop failures, hurricanes, earthquakes, smallpox, etc. Its why an event like the Salem Witch Trials could spin so far out of control; people held onto a cultural anxiety that gave permission to target others with perceived moral faults. Eventually judgement day would arrive, and the increase in providential events would harken it, but each spiritual crisis would push the community closer to God to earn salvation when the day arrived, or it failed and showed how unworthy you and your community were. Even fully fledged church members, the most devout of the devout questioned if they deserved to take Communion on a weekly basis.
The problem is that this anxiety takes a lot of effort and energy to maintain. Puritan Massachusetts is often told as three generations, but I prefer to think of it as three phases. The 1630s, the 1660s, and the 1690s each mark specific points on the Puritan spiritual crisis. This also somewhat lines up with generations of ministers (in some cases actual familial generations of grandfathers to grandsons) coming of age and entering the pulpit themselves. There was disagreement between ministers and plenty of debate in sermons and public writing, but it was constrained. Roger Williams and Anne Hutchinson pushed too far and faced banishment. In this first phase of Massachusetts, it was mostly just handling individual dissenters as they pop up.
In the 1660s, it got more complicated. In order to become a church member, you needed to describe your conversion experience- the moment that you knew God selected you to be one of his visible saints on earth. The second generation was less certain and less inclined to seek membership. Ministers saw this a decline in religiousness, but its more like people were so religious, they felt too uncertain about their own standing to consider themselves worthy. Ministers adapted after a lot of debate to accept the Halfway Covenant. This opened up baptism to more people which was a stepping stone to full membership. (Although local politics and gossip did mean grumbling about how it opened the door too wide.)
However, this doesn’t fully solve the problem of declension. It’s a lot to maintain, and while ministers perceive this decline in the 1660s and continue it forward, they assume it means more providential events will occur which requires more anxiety about the colony’s spiritual status. General congregants struggled to maintain the same emotional and devout responses week after week.
At the same time, the third generation is rising and judgement day hasn’t arrived. For decades upon decades, Puritans expect the Second Coming to arrive, but with each generation that it doesn’t, it becomes harder to maintain church attendance. Ministers like Increase Mather and his son Cotton Mather take up the call to prove providential judgment is right around the corner. Throughout the 1670s and 80s, these two men write a lot about God’s involvement in the world. They put out calls to other ministers to fully and excruciatingly document storms, witchcraft, and other forms of disaster to prove to people that they must return to the church. There were people with this religious fervor and zeal in the benches every week, but again, its hard to maintain. This doesn’t mean people stopped being Puritans or believing in their faith, but the level of enthusiasm that the ministers expected of them kept increasing.
Cotton Mather wrote in his history of New England, Magnalia Christi Americana, Cotton writes, "Religion brought forth prosperity and the daughter destroyed the mother." New England, especially in Boston and Salem, saw massive economic changes over the seventeenth century. The Enlightenment and the Scientific Revolution reshaped so much of how people engaged in the world. The religious elite saw these shifts as a declining religiosity.
Worlds of Wonder, Days of Judgement by David Hall talks about popular religion and print culture- average people were able to read and think about religion independently. The Puritan Church was decentralized, and while ministers gathered and debated, they were most accountable to their congregation. As early American religion progressed, the colonies became a hothouse of different strains of Christianity and other faiths because people could seek a church that identified with their own beliefs rather than follow the strict ideology of a minister. The most published and prominent ministers saw this as a decline and framed colonial Massachusetts as a tragedy where religion lost, but very few people came anywhere near atheism. Its merely that those whose power weakened the most with the fracturing of religion and who failed to adapt called it declension.
I’m not going to break the 20-year rule and draw parallels to today, but when we’re given a narrative about religiosity, a past example like this can show how its often more complicated and that there are distinctions between belief and affiliation that isn’t inherently clear in numbers or polling. A Puritan that isn’t as extreme as Cotton Mather isn’t a non-Puritan, but that person and Mather both needed to navigate a world where religion’s meaning was constantly evolving.
For more- I recommend Hall’s book in particular (as well as his other works), Butler’s Awash in a Sea of Faith (except chapter 5), Staloff’s The Making of an American Thinking Class, Winship’s Hot Protestants: A History of Puritanism in England and America as well as his article “Were There Any Puritans in New England?”, and Winiarski’s Darkness Falls on the Land of Light. Catherine Brekus’ Sarah Osborn’s World also fits in with a lot of this.
I would personally dispute your hypothesis and say at best it’s very western centric, however that’s an area for sociologists and anthropologists to discuss.
Regarding widespread atheism in the pre modern world an interesting time and place to examine is North India in the mid to late first millennium BCE. During this time the states of the Ganges basin experienced widespread disruption from foreign invasion, conquest, and a transition from small scale kingdoms to large empires. These disruptions appear to have led many people to challenge the power of India’s traditional Brahmin priestly caste and their authority based on the Vedic texts and Vedic deities.
The period saw a flowering of religious diversity and a multiplicity of new religious schools appearing, some forming part of what we today call Hinduism, some falling outside it. One thing many of these movements shared was their adoption of renunciation, where members were expected to give up their worldly possessions in pursuit of a higher purpose.
Probably the most famous movements to emerge from this time are Buddhism and Jainism, which are often seen as unusual in the west because of their rejection of a personal deity. However it is important to remember that Siddartha Gautama, the Buddha presented his teachings as a middle way between traditional Bramanic religion and materialism.
The materialists in question were the Charvakas, who seem to have been a very popular movement. Information about them is sketchy and often comes through people who were debating them, such as the dialogues of Siddartha, but they seem to have been a genuinely atheistical school which attracted a widespread following. Charvaka survived into the Middle Ages and in the 8th century a Jain monk Acharya Haribhadra Maharaj described it unambiguously as having "no God, no samsara, no karma, no duty, no fruits of merit, no sin."
There appear to have been Charvakas or people with similar beliefs the court of the Mughal Emperor Akbar in the 16th century where they are recorded as one of the many religions and philosophies invited to debate and discuss eternal truths.
While I wouldn’t say India in 2600 BCE fits your description as an atheistic society, it does seem to have been a place where religious orthodoxy was increasingly and successfully challenged, and many people (particularly the merchant classes) were willing and eager to contemplate radical new ideas, while the government authorities were mostly tolerant of this widespread questioning. The fact that Buddhism which at best denied the power and ultimate authority of India’s traditional Gods went from a fringe sect to the State religion of the Maurya Empire in around 2-300 years indicates how radically the situation changed.
Sources
The Hindus by Wendy Doniger Chapters 1-5
Do Hindus Worship Many Gods? Lecture by Dr Nick Sutton University of Oxford, Centre for Hindu Studies
The Naturalistic Tradition of Indian Thought by Dale Riewe
Classical Indian Philosophy by Sue Hamilton