Christianity

Christianity was one of the major religions of Earth. It centred around the teachings of the religious figure Jesus Christ. Humans would come to base their calendar for much of their history on the estimated year of Christ's birth. 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 celebrated by 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 First 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)

One Time Lord assumed the persona of a Monk in order to alter history in 1066 England. (DW: The Time Meddler)

Christian leaders fought a series of Crusades in the 12th and 13th century, in which the First Doctor and his companions became involved. (DW: The Crusade)

In the 16th century, western Christianity split into two groups: the main group, which became known as the Catholic Church; and the splinter group, who called themselves Protestants. The two groups came into conflict on several occasions, one notable example being the S. Bartholomew's Day Massacre. A key figure in the events leading up to this massacre was the Abbot of Amboise, who bore a striking resemblance to the First Doctor. (DW: The Massacre of St Bartholomew's Eve)

A group of Christian monks in Scotland turned away from their religion, instead harbouring and worshiping a Lupine Wavelength Haemovariform which had landed there. (DW: Tooth and Claw)

The Master masqueraded as a Christian priest as part of his plan to summon Azal. (DW: The Dæmons)

The Doña Arana was a devout Christian living alone in 1980s Spain. She was killed by Shockeye of the Quawncing Grig after her prayer, and Chessene of the Franzine Grig attempted to telepathically extract information about Earth from her dead mind. Chessene noted that Arana's mind was mostly full of her religion, which Shockeye dismissed as "the beliefs of primitives." (DW: The Two Doctors)

The Gorgon was harboured by an order of Christian nuns for generations. (SJA: Eye of the Gorgon)