Must a new pope change his name?

Is there some canon law or any Church law whatsoever that forbids a Cardinal elevated to the Bishop of Rome (Pope) from using his real name while he holds the office?

To the best of my knowledge, there is no law that says a pope must change his name when he is made pope. So far as I know, it is only ancient custom that he does. The custom started in the sixth century with Pope John II who is said to have had the given name of Mercury, the pagan Roman messenger god, and so changed it to a Christian name that better suited his Christian office.

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