Since the ninth century the filioque has been the most contentious theological issue dividing the Western and Eastern Churches. All attempts at resolving the dispute, including the ill-fated Council of Ferrara-Florence (1438--39), ultimately failed because by the eleventh century (if not earlier) the two halves of Christendom had developed theologies of the procession that appeared to be diametrically opposed: either the Spirit proceeded from the Father alone (as the Greeks held) or from the Father and the Son (as the Latins maintained). Yet there was one Church father whose writings offered a way out of this impasse: Maximus the Confessor (c. 580--662).
In his Letter to Marinus (PG 91, 136), Maximus claimed that in using the filioque the Romans, "do not make the Son the cause of the Spirit, for they know that the Father is the one cause of the Son and the Spirit, the one by begetting and the other by procession, but they show the progression through him and thus the unity of the essence." This seemingly irenic text was brought forward at Florence several times, but, read solely within the context of the Photian-Carolingian dialectic, the Letter to Marinus remained merely another proof-text---the Greek anti-unionists viewing it as a clear demonstration of the Latins' heresy (since Laetentur Caeli did attribute causality to the Son), the unionists believing it an apology for filioquism.
Yet I maintain that the Letter to Marinus , properly understood, provided the hermeneutical key to resolving the ancient question of the filioque , and that even in the fifteenth century there existed a school of Byzantine trinitarian theology capable of providing this interpretation. Seen as a clear explication of Maximus's own trinitarian thinking and the consensus patrum as it existed in the seventh century (i.e., the procession of the Holy Spirit from the Father and his eternal flowing forth through the Son), the Letter to Marinus offered the Florentine delegates, and continues to offer today, the best way of reconciling East and West and of establishing (or re-establishing) a genuinely ecumenical understanding of the procession of the Holy Spirit.
