Here is Karl Barth, writing after the first session of Vatican II, comparing the council to the (mainline Protestant) World Council of Churches:
But why is it that the voice of Rome made such a far greater impression than the voice of Geneva on the world…? Was it only because of the obviously greater historical and political halo which Rome possesses? Is the reason not also the fact that in the encyclical the same things were not only talked about but also proclaimed, that Christianity and the world were not only taught but also summoned unreservedly and bindingly with an appeal to the highest authority, that they received not only advice and admonition but also directives, in short, that the encyclical had more the character of a message than our previous ecumenical proclamations, in spite of its extensive use of terms and concepts taken from natural law? I think that our side, lacking this degree of natural law, could actually speak in this manner much more clearly. But at the present I do not yet see that we have done so. And therefore I am afraid that with respect to the external world, precisely in this decisive present of ours, we might be left far behind by a papal church that is making dynamic recovery.
“Thoughts on the Second Vatican Council,” in New Theology no. 1, eds. Martin Marty and Dean Peerman (New York: Macmillan, 1964), 118. Emphases original.
Originally published in the WCC’s Ecumenical Review, July 1963.