God is Real, unless declared an integer! That's what this hoody says.
This refers to the FORTRAN language, in which undeclared variables can be typed based on the first letter of the variable name in question. In the case of 'God', this means that (under these implicit typing rules), the God variabled would be declared as real (single/double precision floating number).
If you're a FORTRAN user already, you probably already got the joke, but no doubt it is even funnier when explained in a paragraph of pure programming, mathematical geekery. :D