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). This pontiff 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.