Atheism

Atheism was the lack of belief in the existence of a god or gods.

The Doctor
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) Regardless, the Tenth Doctor's perceptions about religion were altered upon meeting the Beast, an entity that claimed itself to be Satan. (TV: The Satan Pit)

Other Time Lords
As Lady President of Gallifrey, Romana II commented, "There are no gods". (AUDIO: Neverland)

From Earth
Erimem once stated that she did not believe in the Egyptian gods, (AUDIO: The Eye of the Scorpion) though she later called upon Osiris when the Fifth Doctor was in need of healing. (AUDIO: The Bride of Peladon) 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) Though she was an atheist, she did believe in an afterlife, but she found her seemingly-conflicting belief odd. (PROSE: Falls the Shadow)

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)

Elsewhere
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)

When he was five years old, Skagra decided that God did not exist, and deduced that that meant there was a "vacancy." (PROSE: Shada) The Daleks claimed gods didn't exist. (AUDIO: The Final Phase)