Q. How did the custom of popes choosing new names arise?
A. First some background: The earliest popes did not choose a different name once they took the papal throne. The first to do so was Pope John II (died 535). He was a Roman by birth, who was named Mercurius after the Roman god Mercury. He took the Christian name John, because he thought a pagan name would be a dishonor to the papal office.
Pope John III (died 574) may also have changed his name, but we don’t know for sure. In the latter part of the tenth century, four more popes chose new names for themselves upon ascending the throne. The custom was firmly established by the middle of the eleventh century.
The choice of name belongs to the pope himself. Though there is no canon law requiring that someone take a different name upon becoming pope, the tradition now has the weight of centuries behind it. In addition, the practice has a certain usefulness, since it allows each new pope to make a kind of statement about his hopes and intentions for his papacy.
Reasons for the names chosen have varied. Popes John II and John III apparently took that name to honor their martyr predecessor, Pope St. John I. When a German named Bruno was named pope in 996, he probably took the name Gregory V as a way to reassure the Romans that even though he was a “foreigner,” he would serve them faithfully as earlier, Italian, popes had done.
In our own day, St. John XXIII and Blessed Paul VI were the popes who presided over the Second Vatican Council. As a way of honoring them and associating themselves with the work of that council, Paul's successor took the name John Paul (the first instance of a pope taking a double name), and John Paul'’s successor chose to be called John Paul II.
Pope Benedict XVI probably took that name for several reasons. St. Benedict (c. 480 – c. 547) is the patron protector of Europe, and this pontiff seemed especially concerned about that continent’s threatened spiritual welfare. He also spoke of the need for the Church to be a creative spiritual minority within the world as the Benedictine and other religious orders have been. Also, the previous Benedict was a peacemaker, as this Benedict hoped to be.
One interesting note: A Roman named Peter was elected pope in 1009, but he changed his name to Sergius IV. His reason? Since the first pope, St. Peter, held a unique position as the “Prince of the Apostles,” Sergius thought there should not be a “Pope Peter II.” To this day, the name Peter has never been taken by another pope.