Complex sentence: As the reign of the pope has conventionally been from election until death, papal resignation is an uncommon event; only five popes have unambiguously resigned with historical certainty, all but one between the 11th and 15th centuries.
Split into simpler sentences: As the reign of the pope has conventionally been from election until death, papal resignation is an uncommon event. Before the 21st-century, only five popes unambiguously resigned with historical certainty, all between the 10th and 15th centuries.