The Knights of the Lambda Calculus is a semi-fictional organization of expert Lisp and Scheme hackers. The name refers to the lambda calculus, a mathematical formalism invented by Alonzo Church, with which Lisp is intimately connected, and references the Knights Templar.
There is no actual organization that goes by the name Knights of the Lambda Calculus; it mostly only exists as a hacker culture in-joke. The concept most likely originated at MIT. For example, in the Structure and Interpretation of Computer Programs video lectures, one of the lecturers presents the audience with the button, saying they are now members of this special group. However, according to the Jargon File, a "well-known LISPer" has been known to give out buttons with Knights insignia on them, and some people have claimed to have membership in the Knights.
In popular culture
A group that evolved from or is similar to them, called The Knights of Eastern Calculus, make a major appearance in the anime series Serial Experiments Lain. References to MIT professors and other American computer scientists are prominent in Episode 11 of the series. At one point in the anime, Lain is seen with code displayed on her handheld device that appears to be Lisp.