Frederick grew up in Sicily as his mother was the daughter of the Norman King of Sicily. The court was full of educated Arabs and Frederick got a Muslim view of Christianity as well as a Christian view of Islam.

This heretic upbringing culminated in a lifelong struggle between emperor and pope after Frederick II had secured the imperial crown. He got excommunicated by Pope Gregory IX but Frederick, unconcerned, embarked on a Crusade to win back Jerusalem from the Muslims.

On his return he reconquered Sicilian lands which had been taken by papal troops. He proposed a union of European princes against the ambition of the pope to become ruler of Europe. The emperor got excommunicated a second time and this time Frederick proposed to his fellow princes a general confication of church property.

In the end Rome defeated him, but the struggle left the prestige of the Papacy so badly shattered that Papal domination declined.

The Medieval Sourcebook:
Statute in Favor of the Princes