This has little to do with English language and usage: it is entirely to do with the conventions of Fortran, a computer programming language.

Fortran has two kinds of variables: Integer variables (that hold an integer number) and real variables (that hold a real number).

By default, any variable whose name begins with the letters I,J,K,L,M or N is integer, otherwise it is real. So a variable called GOD would by default be a "real variable" - in short, "GOD is real".

The joke is that this might be presented as an argument for the existence of God.