From The Jargon File (version 4.4.7, 29 Dec 2003) :
Knights of the Lambda Calculus
n.
A semi-mythical organization of wizardly LISP and Scheme hackers. The name
refers to a mathematical formalism invented by Alonzo Church, with which
LISP is intimately connected. There is no enrollment list and the criteria
for induction are unclear, but one well-known LISPer has been known to give
out buttons and, in general, the members know who they are....