The rejection derived from the loss of prestige Charlemagne had suffered at not being represented at the Council of Nicaea, which led him to consider the council unecumenical. The Council of Frankfurt possessed a memorandum about iconodulism, which had been produced previously by Frankish theologians on the order of Charlemagne concerning the Byzantine iconoclastic controversy: the Libri Carolini.
The rejection stemmed from Charlemagne's diminished standing after being excluded from the Council of Nicaea, leading him to believe it was not a truly ecumenical gathering. The Frankfurt Council had previously produced a document on iconodulism, which was commissioned by Charlemagne and related to his efforts in the Byzantine iconoclastic controversy: the Libri Carolini.