>“What is the oldest culture in existence that has remained largely unchanged since it originated?”

Then the answer would be the Indigenous Australians.

Their oral stories, the Dreamtime, speaks of prehistoric fauna, now extinct!

The Christianization of the Greeks is not a spontaneous event, it is a transition that lasted for basically 6 centuries, from the teachings of the Apostle Paul to the last examples of Polytheists mentioned in the Eastern Roman Empire (the one in the 10th century AD by the Roman Emperor Constantine Porphyroghenetos is a mistake, he confused the nationaly Hellenes Mainotes with the religiously Hellenes Melingoi Slavs).

With that in mind, it is important to also remind how Greek Polytheism was not something static. In the 7th century BC, during the time of Solon, you would find Athena being worshiped in Athens, but 6 centuries before that, in the 13th century BC, you would find Potnia instead (which became Potnia of Athens, and from there formed Athena).

> the Greeks became Christians (change of culture).

This is not a change of culture, but a change of religion.

These seem like a good indicator. In 19th century Western Europe the population percentage that would be engaged in the military was about 2%, with the exception being the militarized Kingdom of Prussia with a percentage of 3,5% (if I remember correctly).

Given how these societies are far older, so less manpower would be needed elsewhere, a reasonable percentage could be 5%. Therefore, Rameses II's 100 thousand could easily translate into 2 million people, which seems a reasonable figure for the time (since the population fell in the Bronze Age Collapse, and how in Ptolemaic Egypt the figure was 3 million people).