What is the social main idea of the Ancient Greeks?

Answer 1

There was more than one revolutionary social concept developed by the Greeks, and those concepts changed the world:

1. A "public" religion that did not compete with, or replace, private religious beliefs or practices (this was the "glue" that bound together the fiercely independant Greek city-states into a cohesive whole)

2. The concept of natural causes being the explanation for what happens in the natural world (as opposed to divine retribution or reward)

3. The elevation of individuals (excepting, of course, women and slaves) to "citizens' who participated in public affairs, including law-making

4. Ethics - the idea that there was a way to tell right from wrong without divine authority, decided my rational humans

5. Secular law

Answer 2

Democracy was developed in Athens, so it would be the idea that all citizens could participate, in some form, in their own governing.