“Theology is never any help; it is searching in a dark cellar at midnight for a black cat that isn't there. Theologians can persuade themselves of anything.”

This is attributed, with an expression of doubt as to its correctness, in Mathematics, Our Great Heritage: Essays on the Nature and Cultural Significance of Mathematics (1948) by William Leonard Schaaf, p. 163; also attributed in Pi in the Sky : Counting, Thinking and Being (1992) by John D. Barrow. There are a number of similar expressions to this with various attributions, but the earliest published variants seem to be quotations of Lord Bowen:
When I hear of an 'equity' in a case like this, I am reminded of a blind man in a dark room — looking for a black hat — which isn't there.
Lord Bowen, as quoted in "Pie Powder", Being Dust from the Law Courts, Collected and Recollected on the Western Circuit, by a Circuit Tramp (1911) by John Alderson Foote; this seems to be the earliest account of any similar expression. It is mentioned by the author that this expression has become misquoted as a "black cat" rather than "black hat."
An earlier example with "hat" as a learned judge is said to have defined the metaphysician, namely, as a blind man looking for a black hat in a dark room, the hat in question not being there Edinburgh Medical Journal, Volume 3 (1898)
With his obscure and uncertain speculations as to the intimate nature and causes of things, the philosopher is likened to a 'blind man in a dark room looking for a black cat that is not there.'
William James, himself apparently quoting someone else's expression, in Some Problems of Philosophy : A Beginning of an Introduction to Philosophy (1911) Ch. 1 : Philosophy and its Critics
A blind man in a dark room seeking for a black cat — which is not there.
A definition of metaphysics attributed to Lord Bowen, as quoted in Science from an Easy Chair (1913) by Edwin Ray Lankester, p. 99
A blind man in a dark room looking for a black cat which isn't there.
A definition of metaphysics attributed to Lord Balfour, as quoted in God in Our Work: Religious Addresses (1949) by Richard Stafford Cripps, p. 72
A philosopher is a blind man in a dark room looking for a black cat that isn't there. A theologian is the man who finds it.
H. L. Mencken, as quoted in Peter's Quotations : Ideas for Our Time (1977) by Laurence J. Peter, p. 427
A metaphysician is like a blind man in a dark room, looking for a black cat — which isn't there.
Variant published in Smiles and Chuckles (1952) by B. Hagspiel
Misattributed

This quote was instead first mentioned in a 1931 book titled “Since Calvary: An Interpretation of Christian History” by the comparative religion specialist Lewis Browne.
Disputed

The Rum Tum Tugger
Old Possum's Book of Practical Cats (1939)

TV Series and Specials (Includes DVDs), Mind Control (1999–2000) or Inside Your Mind on DVD

The Tragic Sense of Life (1913), Conclusion : Don Quixote in the Contemporary European Tragi-Comedy

Source: The War of Gods: Religion and Politics in Latin America (1996), p. 23 http://books.google.com/books?id=gyOHaZFpvL8C&pg=PA23