With Rosser in 1936, he showed that λ-expressions that reduce to an irreducible normal form have a unique normal form. In that same year he showed the impossibility of decided whether such a normal form existed.
Church's thesis, which he states as a definition: "An effectively calculable function of the positive integers is a λ-definable function of the positive integers."
Again in 1936, Kleene and Church showed that computing normal forms have the equivalent power of the recursive functions of Turing machines. And thus the Church-Turing thesis was born: Everything computable is computable by a Turing machine.
The λ-calculus also set the stage for many of the functional programming languages like lisp and scheme.
Alonzo Church passed away on August 11, 1995 in Ohio.