Atheism

Atheism is the belief in the non-existence of a god or gods. Atheism is not to be confused with agnosticism, the belief that it is not possible to know whether or not a god or gods exist.

In-universe
The First Doctor told Galileo Galilei that he was an agnostic and fully expected to be an atheist by the end of his travels. (PROSE: The Empire of Glass). The Seventh Doctor later said that he had "abjured religion". (PROSE: All-Consuming Fire) The Ninth Doctor spoke disparagingly of humanity's willingness to "believe in something invisible". (TV: Aliens of London)

Sarah Jane Smith was an atheist. (AUDIO: Dreamland) While drinking in a beer tent on the planet Crex, Bernice Summerfield asserted, "There is not a God!" (PROSE: Human Nature) As Lady President of Gallifrey, Romana II commented, "There are no gods". (AUDIO: Neverland) River Song commented to a Roman commander, "You've been a soldier too long to believe there are gods watching over us." (TV: The Pandorica Opens)

The people of Elbyon were unable to conceive of gods or an afterlife because to do so would have been extremely dangerous in the presence of the planet's "magic" technology. (PROSE: The Sorcerer's Apprentice)

Atheism may or may not correlate with a lack of religious belief or practice (for example, Buddhism has been described as a non-theist religion). Wilfred Mott used the expression "God save their souls" when southern England was destroyed in an alternate timeline, but said he seldom entered churches because they were "too cold". (TV: Turn Left, The End of Time) The Eleventh Doctor described his companion Rory Williams as "not religious or superstitious". (TV: The God Complex)

Real life
Various Doctor Who writers and actors have been self-identified atheists, including Dave Stone, Russell T Davies, Mark Gatiss and Sylvester McCoy. Douglas Adams described himself as a "radical atheist". Biologist Richard Dawkins, who made a cameo appearance as himself in TV: The Stolen Earth, is an extremely well-known advocate for and writer on atheism.