Franz Bibfeldt is a fictitious theologian and in-joke among American academic theologians.
Bibfeldt made his first appearance as the author of an invented footnote in a term paper of a Concordia Seminary student, Robert Howard Clausen. Clausen's classmate, Martin Marty, was struck by the name and Bibfeldt became a running joke for Martin and his friends. In 1951, Marty's review of Bibfeldt's The Relieved Paradox was published in the Concordia Seminarian, to the bewilderment of the Concordia faculty. When the ruse was uncovered, Marty's fellowship to study overseas was revoked, and he instead enrolled in the University of Chicago, where he spent the rest of his academic career; he thus credits Bibfeldt as the German theologian who had the greatest influence on his work.
Since then Bibfeldt scholarship has greatly expanded, though the preponderance of work has come out of the University of Chicago, where there is a Donnelley Stool of Bibfeldt Studies. Bibfeldt's bibliography includes his doctoral thesis, "The Problem of the Year Zero"; his response to Søren Kierkegaard's Either/Or, entitled Both/And, as well as the subsequent reconsideration Either/Or and/or Both/And; and his argument for the Mesopotamian origins of baseball, The Boys of Sumer.
Most of the scholarship to date is collected in The Unrelieved Paradox: Studies in the Theology of Franz Bibfeldt (ISBN 0-8028-0745-3) edited by Marty and Jerald C. Brauer, which includes a discussion of "Proofs of the Existence of Franz Bibfeldt."
A Swedish parallel is the fictitious theologian Elof Sundin at Uppsala university since the early 1960s - see the article in the Swedish Wikipedia: sv:Elof Sundin.