"Under St. Peter's" is a short story by Harry Turtledove, first published in The Secret History of Vampires, edited by Darrell Schweitzer, DAW, 2007. It was reprinted in By Blood We Live, edited by John Joseph Adams, Night Shade Books, 2009, and We Install and Other Stories, 2015.
Sure to become one Turtledove's most controversial stories, "Under St. Peter's" tells how Jesus, rather than dying on the cross, instead became a vampire, and was imprisoned in St. Peter's Basilica, guarded by a secret cadre of priests. The viewpoint narration alternates between the vampire Jesus, a somewhat deranged creature given to long periods of thirst, and the newly anointed Pope Benedict XVI, unnamed in the story but described unmistakably. Benedict is taken before Jesus shortly after becoming pope in 2005, and is subject to Jesus' bite, a tradition stretching back to St. Peter.
In the We Install collection, Turtledove says that this is his rare concession to "secret history", a genre which he is usually uninterested in writing.