Within the Church, there is no strict doctrine or catechism that requires a newly elected Supreme Pontiff to take on a new name after their inauguration ceremony. In fact, for hundreds of years a new pope would simply continue to use their given birth name. Today, after the counting of ballots produces a consensus candidate, a papal conclave will ask of the new pope “By which name will you be known?” However, if taking on a papal name is not a requirement, why do they change their names?
The tradition first started in the year 533 with Pope John II taking on a new moniker. His birth name was Mercurius, his parents having named him after Mercury in the Roman mythology. Upon his election, he thought it would be inappropriate of the Holy See to have a supreme pontiff in office with the name of a pagan god. He took his papal name after Pope John I, who was martyred just seven years earlier.
For over four hundred years, his successors once again returned to using their birth names. However, in December of 983 Pietro Canepanova was elected pope. Not wanting to disrespect or liken himself to Saint Peter, the first Pope, he took on the papal name of Pope John XIV. John XIV’s successor, Giovanni di Gallina Alba, retained his birth name and became John XV.
Following the papacy of John XV, newly elected pontiffs began to use papal names somewhat regularly. Generally, a regnal name was taken when their birth name was distinctly non-Roman, such as a Germanic name. Other times, a new pope would choose a name to honor a predecessor. For example, Pope Saint John Paul II chose his name in honor of John Paul I, whose papacy lasted only 33 days.
The last pope to retain their birth name was Pope Marcellus II, born Marcello Cervini degli Spannochi, in 1555. After his papacy, all of his successors took on a new papal name. However, until the election of Pope Francis, there were no new papal names since the year 918. Names were simply reused, with the most popular being John, used over 21 times. Pope Francis is one of the 44 popes to have a unique papal name.