Pope John XXIV was head of the Roman Catholic Church and Sovereign of Vatican City before he was taken in the Rapture.
It is mentioned in Tribulation Force that he reigned for only five months after the death of an unnamed Pope. Though leader of the Catholic Church, his personal belief system leaned towards the teachings of Martin Luther, a founder of Protestantism.
He was succeeded as Pope by Peter Mathews, who renamed the office as Pontifex Maximus and eventually abolished Roman Catholicism (among other religions) in favor of Enigma Babylon One World Faith.