In 533 a priest called Mercurius was elected as the 56th pope. He did not want to use the name of a pagan god, so he selected John II as his papal moniker, becoming the first pope to change his name. The idea caught on, but it wasn’t until the 10th century that it became routine. Pope Marcellus II in 1555 was the last pope to use his given name.
The practice has even more ancient roots. Old and New Testament figures found themselves with new names after times of great spiritual transition. Rev. Stephen Bevans, a theology professor at the Catholic Theological Union in Chicago made the point:
The tradition of choosing a new name actually hearkens back to the Bible, Bevans said. For instance, after a vision of an angel wrestling with him, old testament character Jacob renamed himself Israel, which means “contended with God.” And the first leader of the church, St. Peter, was known as Simon when he was one of Jesus Christ’s apostles, Bevans said.