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 Jesus Christ's birth (TV: Voyage of the Damned) and his resurrection. (TV: 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. (TV: The Satan Pit)
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 St 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. (TV: The Massacre of St Bartholomew's Eve)
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." (TV: The Two Doctors)