I don't have a lot of time. Now, I'm mainly just thinking out loud. I usually try to say something cute or clever, but there's no time for this today.
I think the best book to read at the outset of a thorough study of Barth's work is The Essence of Christianity (Das Wesen des Christentums, 1841) by Ludwig Feuerbach. The translation I'm looking at was done by George Elliot -- yes, that George Elliot -- which, I'm assuming, was the first translation into English.
(Apparently, Elliot enjoyed promulgating controversial texts, for she also translated Strauss' Life of Jesus. There are so many Strauss' out there in our cultural history: This is the one who put forward the idea that the miraculous material in the gospels is "mythological." Interestingly, according to this source, Elliot published the Feuerbach translation under her actual name, Mary Ann Evans. James Luther Adams, more recently, also translated Das Wesen, but somehow it seems cooler to carry around a book with Elliot's name on the cover. No disrepect is meant to Dr. Adams.)
I don't have any new groundbreaking insight into Feuerbach, whom most theology and philosophy students will have encountered, but I am rereading this work to refresh my memory on the basic argument. which goes like this: Theology is anthropology that doesn't realize this about itself. Theology is anthropology, perhaps, with a bad conscience. I think Feuerbach, were he alive, might level the latter against many theologians working today.
Feuerbach offers the classic modern reductionist account of religion. Religious discourse (theology) represents an alienation of human life and thought from its ownmost proper concern -- humankind itself. According to Feuerbach, all religious doctrines are projected from human ideals and from concrete realities, hopes and aspirations of human experience. Christian doctrines have true referents, but the referents aren't what theology claims them to be: "God" is a hypostasized projection of ideal human characteristics. Feuerbach's thought seeded the major 19th-20th century hermeneuticists of suspicion, that trinity beloved by critical theorists and angry undergrads alike -- Marx, Nietzsche and Freud.
A quick trip to wikipedia would yield all this info. So why do I bother with this stuff? Well, like I've said, I'm obsessed with Barth -- who quite possibly was less obsessed with Feuerbach than I'm supposing, but we'll see. And my thesis is that we need to understand Feuerbach's claims, various possible modern responses to his claims and Barth's assessment of these responses, to get a clear picture of some of the bigger moves in Barth's thought. So I'm relaying some groundwork here, hoping maybe to hit some new gem I never saw before in the dirt.
Then, once I get a better handle on Feuerbach, I might proceed thusly: Flashback to (the early) Schleiermacher, then maybe a quick look (if that were possible) at some Luther. It may be, as others have suggested, that there is potentially a problem in how Luther construes the divine-human relationship which Feuerbach has exploited. At this point, just maybe, I could construct a sort of ideal typology of possible reponses to Feuerbach.
I think this may take a little while. But, fortunately, my Christmas bonus is not tied to my theological work.