The greatest contribution of ancient Greeks to humanity was the invention of "Orthos Logos", which can be translated as "reason", "logic'', ''orthological reasoning'', ''rational thought'' etc.
Heracletus, Epicurus, Lucretius, Democritus, Leukippus were some of the founders of reason. As of no surprise, most of them denied the existence of gods, or at least their intervention in human matters.
It was the coming of Christianity that marked the end of Logos and its substitution by the ''knowledge by divine revelation'': if God granted us all the knowledge we need, then why employ reasoning? So, that marked the end of the ancient world and the start of the Middle Ages. Islam in the 6th century was just another repetition of Christianity in that matter.
It took some one and a half millenium for mankind to discover once again the ancient art of Reason. This procedure started slowly after Renaissance but mostly came on screen during the Age of Enlightenment. The 19th century theory of Evolution was a grand manifestation of the triumph of Reason over religious superstitions.
In the 20th century, alas, we saw a revival of irrational thought, with all this science denial and ''new-age'' religions, and cults like, erm, you know :)
What the 21st century will bring in this fight of reason against irrationality, we will see.