Focio
The word "Filioque" was first added to the Creed at the Third Council of Toledo (589) and its inclusion spread later throughout the Frankish Empire. In the 9th century, Pope Leo III, while accepting, like his predecessor Pope Leo I, the doctrine, tried to suppress the singing of the "Filioque" in the Mass of the Roman rite. In 1014, however, inclusion of "Filioque" in the Creed was adopted in Rome. Since its denunciation by Photios I of Constantinople, it has been an ongoing source of conflict between the East and West, contributing to the East–West Schism of 1054 and proving an obstacle to attempts to reunify the two sides.