Christianity
Christianity was one of the major religions of Earth. It centred around the teachings of the religious figure Jesus Christ, who was born in (approximately) 1AD. The Doctor was apparently present at his birth (DW: Voyage of the Damned) and his death/resurrection (DW: Planet of the Dead). These two events were responsible for the Earth holidays of Christmas and Easter, respectively. Christianity was one of countless religions throughout the universe to have a devil figure inspired by the Beast. (DW: The Impossible Planet/The Satan Pit)
The Doctor met some early Christians while visiting Byzantium in 64. (PDA: Byzantium!)
When Christian missionaries tried and failed to exorcise Saul, they built a church on the site, then declared him an angel. (NA: Timewyrm: Revelation)
Christian leaders fought a series of Crusades in the 12th and 13th century, in which the Doctor and his companions became involved. (DW: The Crusade)
In the 16th century, Christianity split into two groups: the main group, which had become known as the Catholic Church; and the splinter group, who called themselves Protestants. The two groups came into conflict on several occasions, most notably in the St. Bartholomew's Day Massacre. (DW: The Massacre of St Bartholomew's Eve)
A group of Christian monks in Scotland were turned away from their religion, instead harbouring and worshiping a Lupine Wavelength Haemovariform which had crashed there. (DW: Tooth and Claw)
The Master masqueraded as a Christian priest as part of his plan to summon Azal. (DW: The Dæmons)
The Gorgon was harboured by an order of Christian nuns for generations. (SJA: Eye of the Gorgon)
See also
