Without further ado, here it is: from letter 47. Bold is me:
[Y]ou have failed to enter into (latent but radical) debate with modern philosophy and naively adopted the older ontology from patristic and scholastic dogmatics. What you say (and often only want to say) is beyond your terminology, and a lack of clarity and sobriety is frequently the result. You have a sovereign scorn for modern work in philosophy, especially phenomenology. What point is there in saying occasionally that the dogmatician must also be oriented to philosophical work if the presentation finds no place for this orientation…? It seems to me that you are guided by a concern that theology should achieve emancipation from philosophy. You try to achieve this by ignoring philosophy. The price you pay for this is that of falling prey to an outdated philosophy.
It is right that dogmatics should have nothing whatever to do with a philosophy insofar as this is systematic; but it is also right that it must learn from a philosophy that is a critical (ontological) inquiry. For only then does it remain free and make use of philosophy as a helper of theology; otherwise it becomes the maid and philosophy the mistress. There is no alternative; it must be either maid or mistress. Your planned ignoring of philosophy is only apparent. Naturally lordship or servanthood applies to the forming of concepts. But if dogmatics is to be a science, it cannot avoid the question of appropriate concepts.
This correspondence is interesting because it shows Barth working through some central methodological issues, with Bultmann’s help, at the moment when he was turning to constructive (positive) dogmatic work. Bultmann’s warning here has as much force for us today as it did for he and Barth back then, and I think the story of Barth’s theology is of trying to do justice to this concern. Different interpreters might well have different answers to whether or not Barth finally succeeded (same for Bultmann, for that matter). But both this concern to speak for today, and this danger of falling unwittingly back upon a previous and philosophically outdated metaphysics / ontology / epistemology / what-have-you must be consistently held before our eyes. As Barth said decades after this letter from Bultmann, “even the slogan ‘Back to the Reformers,’ cannot promise us the help that we need to-day. ‘Back to…’ is never a good slogan.”