Pope Leo III was pope around the turn of the 9th century, prior to the Great Schism, but during a time when there was already controversy between East and West concerning the nature of the Trinity and the manner in which the Holy Spirit was related to the Father and to the Son. There was already a common practice in many areas of the Western church to include the filioque in the creed when it was recited at Mass, and there had been significant discussion and criticism (at the level of patriarch, emperor, council, and pope) in the last 15 years of the 8th century.
In 808, a Greek monk noticed that the “liturgical books used in a Frankish monastery on the Mount of Olives included the filioque.” He proclaimed them heretical; the Frankish monks appealed to the Pope, who wrote to the emperor, Charlemagne, who called a synod in 809 “with the aim of approving and justifying” this widespread practice. Charlemagne further sent a delegation to the Pope, to persuade him of the doctrine and the change to the creed. Although he agreed with the teaching,
Leo III, however, refused to comply with the second request and insisted on retaining the pure text. In order to make his own position public, he had the text without the Filioque engraved in Greek and Latin on two silver scrolls and these were hung on each side of the entrance to the high altar or Confessio in St. Peter’s.
I think this makes Leo III an apt intercessor for the cause of ecumenical dialogue, particularly between the Roman Catholic and Eastern Orthodox churches. His feastday is June 12.
St. Leo, pray for us!
All quotations taken from Congar, Vol III, p 57.