Pope Innocent III
In the Middle Ages, the pope was the supreme head of the church in England: not until the Reformation, over 300 after Magna Carta, did Henry VIII reject papal supremacy.
Innocent III was elected pope in 1198 and reigned until his death in 1216. His papacy, which coincided almost exactly with King John's reign of 1199-1216, saw repeated attempts to enforce papal authority over secular rulers, campaigns to rid the church of heretics and the Fourth Crusade. In ecclesiastical government, Innocent III's determination to impose his judicial authority over the whole Latin church culminated in the major reforms of the Fourth Lateran Council of 1215.
By the time of his death in 1216, Innocent had set in place the framework for the power of the 13th-century papacy.