Short answers, pulled from the story.
Jean-Paul Sartre defined abandonment as the derivative of atheism in his 1946 lecture L'Existentialisme est un humanisme. He stated that existence precedes meaning and the absence of God became known as abandonment through this specific address.
Martin Heidegger wrote about the abandonment of self before Jean-Paul Sartre defined abandonment regarding higher powers. He derived ideas from Friedrich Nietzsche's work to theorize that distress stems from the lack of distress itself.
Albert Camus, Michel Foucault, and Noam Chomsky form the refrain of well-known atheists following this foundational philosophy. Frank R. Zindler published Ethics Without Gods in 1985 within American Atheists publications arguing ethical behavior results always from the same causes regardless of religious belief.
The Supreme Court case Murray v. Curlett removed reverential Bible reading from public schools and banned oral unison recitation of the Lord's Prayer in those institutions. Petitioners in the case were all atheists who defined their beliefs through philosophical abandonment.