Historians rate Frederick II as a highly significant European monarch of the Middle Ages. This reputation was present even in Frederick's era. Lansing and English, two British historians, argue that medieval Palermo has been overlooked in favor of Paris and London:  One effect of this approach has been to privilege historical winners, [and] aspects of medieval Europe that became important in later centuries, above all the nation state.... Arguably the liveliest cultural innovation in the 13th century was Mediterranean, centered on Frederick II's polyglot court and administration in Palermo.... Sicily and the Italian South in later centuries suffered a long slide into overtaxed poverty and marginality. Textbook narratives therefore focus not on medieval Palermo, with its Muslim and Jewish bureaucracies and Arabic-speaking monarch, but on the historical winners, Paris and London.  Modern medievalists no longer accept the notion, sponsored by the popes, of Frederick as an anti-Christian. They argue that Frederick understood himself as a Christian monarch in the sense of a Byzantine emperor, thus as God's "viceroy" on earth. Whatever his personal feelings toward religion, certainly submission to the pope did not enter into the matter in the slightest. This was in line with the Hohenstaufen Kaiser-Idee, the ideology claiming the Holy Roman Emperor to be the legitimate successor to the Roman Emperors.  20th century treatments of Frederick vary from the sober (Wolfgang Sturner) to the dramatic (Ernst Kantorowicz). However, all agree on Frederick II's significance as Holy Roman Emperor. In the judgment of British historian Geoffrey Barraclough, Frederick's extensive concessions to German princes--which he made in the hopes of securing his base for his Italian projects--undid the political power of his predecessors and postponed German unity for centuries.  However, the modern approach to Frederick II tends to be focused on the continuity between Frederick and his predecessors as Kings of Sicily and Holy Roman Emperors, and the similarities between him and other thirteenth-century monarchs. David Abulafia, in a biography subtitled "A Medieval Emperor," argues that Frederick's reputation as an enlightened figure ahead of his time is undeserved, and that Frederick was mostly a conventionally Christian monarch who sought to rule in a conventional medieval manner.

Answer this question "what was his religion" by extracting the answer from the text above.
Christian