Catholic Church
For centuries, Popes have not used their birth names, but instead use a regnal name. This custom started in the sixth century when a man named Mercury was elected Pope, it was seen as inappropriate to have a Pope named after a pagan god so he named himself John II, in honor of his predecessor, John I; it became customary in the tenth century. Since the death of Marcellus II 1555, every Pope has taken a Papal name.