The question was, "Why does the Pope change his name?" If Christ changing Simon's name is irrelevant, why did you bring it up? The question, nowadays as to why the Pope changes his name, is this: it's a way to signal something, either publicly or to an inner circle or to himself personally. Which is all fine. But as to the tradition of the Pope naming himself, that's still unclear. And I don't know that answer! 😀