At the time of the Council of Trosley (909), West Francia was in great disarray due to the collapse of the Carolingian empire and continuous Viking raids which devastated much of Western Europe at the time. As we can see from the minutes of the council the Frankish bishops believed that the Vikings were a manifestation of God's divine wrath against the corruption of the church, and thus reform and a return to proper canonical order were called for. It has been speculated that this council inspired the Cluniac Reform Movement, given the fact that Cluny Abbey was founded a year after the council took place.
Of special interest to modern Orthodox Christians, is the fact that this council was the first in the West to synodally affirm the filioque clause in the Creed. On pages 87-88 we read the words of the leader of the council Archbishop Heriveus of Reims: "The holy Apostolic See reports that even now the errors and blasphemies of a certain Photius persist in the regions of the East – blaspheming that the Holy Spirit proceeds not from the Son but only from the Father. We exhort your fraternity, together with me, that according to the admonition of the lord of the Roman See, each one of us, having considered the judgements of the catholic Fathers, may bring forth keen arrows of the Mighty One from the quivers of divine Scripture, for piercing the beast of the reborn monster and for crushing the head of the most wicked serpent."
By the early tenth century the papacy seems to have accepted the filioque clause as dogma, despite the anathema proclaimed against this innovation thirty years earlier by the Eighth Ecumenical Council (879-880), with the assent of the Roman Pontiff. At the time of the council (of Trosley), the reigning Pope was Sergius III, who ushered in the era known as the "Pornocracy" (904-964), a time when the popes were influenced strongly by a powerful and corrupt aristocratic family, the Theophylacti, and their relatives and allies. Sergius III himself was of notably ill repute as he had allegedly ordered the murders of his two papal predecessors, and fathered an illegitimate son during his pontificate, who later became Pope John XI. There is thus reason to doubt that Sergius III and his successors Anastatius III and John X were canonical popes. In fact a hundred years later, Patriarch Saint Sergius II of Constantinople removed the names of these three pontiffs from the diptychs for their support of the filioque. It was the same Saint Sergius II who broke communion with Rome in 1014 after the filioque was added to the Creed there for the first time.