The original Nicene Creed (AD 381) said that the Holy Spirit 'proceeds from the Father' and the Eastern Orthodox churches follow that wording to this day. However, in the West the growing tradition was to think of the Spirit as 'proceeding from the Father and the Son' (Latin: filioque) and eventually in AD 589 the ecumenical creed of Nicea was modified by the Catholic Church to include the word filioque ('and the Son'). This controversial move was the sole doctrinal cause of the Great Schism that divided the Orthodox and Catholic Churches (AD1054) and it remains a dividing issue with the Christian Church to this day. This study examines the defense of the filioque clause in four of the leading medieval theologians in the Catholic Church, seeking to show why it mattered so much to them: * Anselm (1033-1109) * Richard St Victor (d. 1173) * Thomas Aquinas (1224-1275) * Bonaventure (1217-1274) The book provides: * An initial chapter providing the historical background to the debate about filioque. * A introduction to the general Trinitarian theology of each influential thinker. * A detailed study of the reasoning behind their support for the filioque clause.