"In 1967, he [Langlands] came up with revolutionary
insights tying together the theory of Galois groups 
and another area of mathematics called harmonic
analysis. These two areas, which seem light years
apart, turned out to be closely related."

— Edward Frenkel, Love and Math, 2013

"Class field theory expresses Galois groups of
abelian extensions of a number field F
in terms of harmonic analysis on the
multiplicative group of [a] locally compact
topological ring, the adèle ring, attached to F."

— Michael Harris in a description of a Princeton
    mathematics department talk of October 2012

Related material: a Saturday evening post.

See also Wikipedia on the history of class field theory.
For greater depth, see Tate's [1950] thesis and the book
Fourier Analysis on Number Fields .