Greece and Rome's civic and philosophical development was the source of how Western Civilization developed. Christianity was then transplanted on top of Western Civilization; it wasn't the source. The author is either mistaken or arguing in bad faith, and makes numerous factually incorrect statements. He argues the Locke "assumed" God given rights, and states that there are no other legitimate sources, despite there being a large body of work in the 3rd century BCE that made logical arguments for treating all of humanity equally. Similarly, in his Meditations, Marcus Aurelius argues that gods or atoms, your first responsibility is to the human race.

The author would have us forget that much of the Enlightenment came from the rediscovery of Greek and Roman knowledge.

The whole article reads as an attempt to convince atheists that they're not really atheist, that all their ideas came from religion and are worthless without religion, completely ignoring the role philosophy had in creating those ideas, long before Christianity was on the scene.