In any case, Amy Marga - who spoke at the recent PTS Barth conference - has a nice paragraph on the foundation of this ecumenical quality of Barth’s thought in her book, Karl Barth’s Dialogue with Catholicism in Göttingen and Münster: Its Significance for His Doctrine of God:
Barth’s curiosity about thinkers outside of the typocal cast of Reformation figures was one of the earliest – but perhaps farthest-reaching – ecumenical move[s] that Barth made. While not ‘ecumenical’ in a contemporary sense of personal dialogue or joint statements on social and doctrinal issues, as ecumenism is carried out today, Barth’s decision to explore the wider history of Christian theology no doubt played a role in how he viewed the scope of his work, and how his work was received by Protestant and Catholic alike. On many levels, Barth’s openness to Christian theologians outside the Reformation was a signal that the wider history of the Church – including many thinkers that were authoritative in the Catholic Church – was as legitimate and useful to the needs and commitments of modern Protestantism as that of the Reformation. His exploration of the wider history of Christian theology bestowed the same value and power upon pre-Reformation (i.e. “Roman Catholic” theology) as that of the Reformation. Such a shift in vision in the early decades of the twentieth century signaled the inevitable weakening of the liberal Protestant stronghold on the interpretation and valuation of history. If the entirety of Christian history was legitimate for informing theology, there was no reason why Protestants and Catholics could not study this history in conversation with one another (28-29).