Atheism is the disbelief or lack of belief in the existence of God or gods, while agnosticism is the view that the existence of God is unknown or unknowable.

While individual philosophical doubt about gods existed in the ancient world (e.g., in Greece and India), organized atheism is a modern phenomenon.

The Age of Enlightenment in the 18th century, with its emphasis on reason and skepticism, was a crucial period for the development of modern non-belief.

Baron d'Holbach was one of the first prominent, openly atheistic writers of the Enlightenment.

The 19th century saw the rise of influential atheist thinkers like Karl Marx, who famously called religion 'the opium of the people.'

Charles Darwin's theory of evolution provided a scientific explanation for the origin of species without recourse to a divine creator, which bolstered non-belief.

Communist states in the 20th century, such as the Soviet Union, officially promoted state atheism and suppressed religious practice.

The late 20th and early 21st centuries have seen the rise of a 'New Atheist' movement, with authors like Richard Dawkins and Christopher Hitchens advocating for a more assertive and public form of atheism.

The legal separation of church and state in many Western countries has created space for non-religious worldviews to flourish.

The history of atheism and agnosticism is closely tied to the history of science, philosophy, and political secularism.