What exactly do you require? Is atheism a necessary component? Industrial labor? A modern centralised state?
If so, then the earliest is really somewhere around the late 18th century; like a different course of the French Revolution.
If some or all of the above are not compulsory, then OTL provides lots of examples of radically egalitarian societies with an emphasis on communality: many societies we label as "tribal", from the Americas over Eastern Africa to Central and Northern Asia, were organised on the principles of common ownership of land and participatory decision-making, where some individuals sometimes distinguished themselves and were revered by others because of some inspiration they had (shamans...) or because of their military prowess in fights against other tribes, but neither of these led to hereditary classes or to great material differences.
A little more complex, we had various classical Indian mahajanapadas with a democratic political outlook and corporative economic structures which had collective control and quasi-religious producer ethos instead of competition and profit-drive. Much later, European guild-led towns sometimes came close to this, but not quite.
Then, there are of course countless religious communities, from Buddhist sanghas to Christian monastic orders, which adhered to radically egalitarian principles. You could count all of Early Christianity into this fold if you wanted to. Later sects like the Paulicians, the Bogumils, and the Hussites wanted to implement this nation-wide, while other groups lingered merely on the verge of heresy by practicisng communal life and ownership without seeking to include everyone, like the Beghines and the Beghards, the Fraticelli etc.
Later, England had the Diggers.
There are tons of examples I have glossed over now, question is: what exactly are you looking for?