Complex sentence: Some 11th century historians believed that there had been a pope named John between antipope Pope Boniface VII and the actual Pope John XV; thusm, the series of Pope John XV to Pope John XIX was mistakenly numbered John XVI to XX.
Split into simpler sentences: Some 11th century historians believed that there had been a pope named John between antipope Pope Boniface VII and the actual Pope John XV. Thus the series of Pope John XV to Pope John XIX was mistakenly numbered John XVI to XX.