"God" was one of the most straightforward names given to the singular powerful entity who purportedly created the universe inhabited by the Doctor and humanity. God also described Himself as He Who Creates or the Creator, and once told Peri Brown to refer to Him as simply Sir. He could also be known as the Almighty.
While others believed in multiple gods, some religions acknowledged only the singular figure of God. One of them, Islam, referred to its conception as Allah. (AUDIO: Escape Room) According to some accounts, God as commonly conceived-of was a real being predating the universe, who had even taken an interest in the Doctor throughout their lives, although from the perspective of his own sphere of existence he was not especially powerful. (PROSE: Power to the People) In other accounts, while the specific religious concept of God was fictitious, it disguised a real truth, with "God" or "Allah" appearing in religious texts as a distorted stand-in for the architect and founder of the Great Houses who brought order to the Spiral Politic. (PROSE: Head of State)
Biography
Creating the universe
According to one account, God was a member of a species from outside the Doctor's universe, to whom the task of creating universes was mundane. He created the Doctor's universe as part of what could be conceived of, in human terms, as "an advanced science lesson". He feared that one of his "classmates" may attempt to steal the universe He had made, so, having placed a heart at the centre of the universe, he added a barrier around it, which no member of His own species could touch, see, or harm in any way.
God also created a Guardian of his own species, and placed him inside the universe he had made. The Guardian could never leave again once he had entered it. To this Guardian he entrusted the Key to Time. Upon entering the universe, however, something went wrong and the Guardian split in two halves, one evil, one good: the White Guardian and the Black Guardian. (PROSE: Power to the People) According to the Scrolls of Gallifrey, the figure responsible for shaping much of the universe who created the Guardians from his essence and entrusted them with the Key to Time was Rassilon. (PROSE: The Legacy of Gallifrey)
According to human religious beliefs, shortly after creating humanity, God gave them a paradise in the form of the Garden of Eden, but they failed him, being cast out of the garden. (PROSE: Halflife)
Contacts with the Doctor
Because the Guardian had split in two, God knew that the White Guardian would not be able to replace the heart of the universe all by himself when the time came, due to the strenuous effort that recovering the scattered pieces of the Key to Time represented. He foresaw (PROSE: Power to the People) that the Fourth Doctor would be in a position to gather the pieces of the Key on the White Guardian's behalf, (TV: The Ribos Operation) and thus decided to influence the Doctor's life.
This started as early as the Doctor and Susan's escape from Gallifrey, where he acted invisibly to jam a guard's staser to give the Doctor the time to get into "the machine". Later, the Doctor's fated death was to suffocate in the void of space near a Ubankan ship, trying and failing to get to his TARDIS, but God enveloped him in a bubble of air and permitted him to survive. He also helped the Doctor combat the threat of Omega, ensuring that he would find his flute in the TARDIS mechanisms at a crucial moment, (PROSE: Power to the People) allowing him to defeat Omega. (TV: The Three Doctors)
The spare batteries given to the White Guardians were "duds", however, rendering the Doctor's original quest for the Key moot. God was forced to contact him again in his sixth incarnation, during his travles with Peri, speaking to him directly as a disembodied voice in his TARDIS. He had the Doctor replace the heart himself. In the process, the Doctor realised that the heart was a mundane battery. Realising that the revelations he had made about the ultimately underwhelming nature and reason for the universe were disturbing to the Doctor and Peri, God then returned them to the TARDIS with no memory of their encounter with God. (PROSE: Power to the People)
In culture
Swears and interjections
With the broad concept of God being so well-known, interjections hyperbolically calling out to God were common, without intent to actually invoke the real entity, nor even necessarily a belief in God on the part of whoever was swearing. Common phrases among humans included "Oh my God", (TV: Eve of the Daleks) "God-speed", (TV: The Day of the Doctor) or simply an aggravated "God!". (TV: The Satan Pit, Eve of the Daleks) Even the Doctor sometimes used such phrases, with the War Doctor notably using the phrase "Oh, for God's sake!" moments before he saved Gallifrey with his other selves. (TV: The Day of the Doctor)
Religion and rituals
When knighting Ian Chesterton, Richard I of England intoned that he was doing so "in the name of God, Saint Michael and Saint George". (TV: The Crusade)