The Synod of Frankfurt, convened under Charlemagne's authority, brought together clergy from the Frankish kingdom, Italy, and England to reject the Byzantine Second Council of Nicaea's stance on icon veneration and to condemn Adoptionism, a Spanish theological movement. The gathering demonstrated Charlemagne's growing role as arbiter of Western Christian doctrine, years before his imperial coronation in Rome.
Charlemagne did not simply attend the Synod of Frankfurt, he directed it, seating himself as the effective judge of Christian orthodoxy across a realm stretching from the Pyrenees to the Elbe. The synod's decrees rejected the findings of the Byzantine-led Second Council of Nicaea, which had endorsed icon veneration, reflecting a mistranslation of Greek theological terms into Latin that made the Byzantine position sound more radical than it was.
The assembled bishops also condemned Adoptionism, a doctrine popular in Spain holding that Christ was God's adopted rather than natural son, a distinction that mattered intensely to churchmen but says something about how thoroughly doctrinal purity had become entangled with Carolingian political consolidation.
The synod mattered less for its theology than for what it revealed: a Frankish king acting as head of Western Christendom's doctrinal authority six years before Pope Leo III crowned him emperor in Rome. It set a precedent for royal involvement in church councils that would echo through centuries of tension between throne and altar.
Key people: Charlemagne