Ummm … hate to disappoint, but the Greeks didn’t abandon their old Gods for atheism and agnosticism. They replaced it with Christianity. As for religion impeding the progress of civilisation - Christianity gave the world the university system which did a brilliant job of organising, supporting, preserving and spreading learning in the Middle Ages and continues to do so today.
All significant hospitals and schools of the Middle Ages and for centuries afterwards, were religious institutes. No one but priests, monks and nuns had the necessary time or energy to devote to the arts of learning and healing.
Who but these same monks and nuns had the patience and dedication to spend the hours (months in fact) that were needed to laboriously make even one single good copy of a book? Their secular neighbours were struggling to survive off the land or fighting for their lives against barbarian invasion.
Monks did all these things as well, but ALSO cared enough about learning and knowledge to preserve it for posterity. And I’m not just talking about the Bible either. Where do you think the great medieval philosopher Thomas Aquinas got his incredible knowledge of Aristotle from if not for the religious scribes who preserved the best SECULAR learning of the Greeks and Romans as well as the religious.
How do you think culture, philosophy and ideas spread so easily from end to end of the Christian world if not for the fact that the Church, which was in every corner of it, had one common language - Latin - which funnily enough also became the language in which scientific and other works came to be transmitted as well. The two pretty much went hand in hand.
I know you’re going to spout Gallileo at me. And you’re right, that was a major stuffup. Is any organisation going to go through 2000 years without a single mistake? But the Catholic Church was also one of the earliest supporters of Darwin’s evolutionary theory. Never had a real problem with his explanation of how humans physically came to be. Darwin’s theory of evolution, by the way, with its ideas of ‘survival of the fittest’ and so on, has caused more trouble in regard to relations between the different races and nations than most religious theories.
Funnily enough, in at least one particular area (race relations) most of the great reformers and agitators against African-American slavery, particularly in Britain, were inspired by strong religious faith and a belief that, as the Bible teaches, there is no distinction in God’s eyes between the races and no such thing as slave or freeman.
As for conflict - World War I and World War II were fights between good modern ‘scientific’ nations in the name of progress (ie control of colonies, natural resources and a position as industrial top dog). Nominally most of the participating nations on both sides were of the same Protestant (or at least Christian) faith, in reality they were then, as they are now, incredibly secular.
These two conflicts by themselves, let alone similar non-religious wars such as Vietnam, way outstrip any killing or violence ever done in the name of religion in scale and ferocity. Who ever in the name of religion obliterated cities in the way London, Dresden and Tokyo (not to mention Hiroshima and Nagasaki) were destroyed in WWII? So much for your ‘civilisation’.