St. Augustine on ad orientem

From Ch. 5 of De Sermone Domini in Monte.

... cum ad orationem stamus, ad orientem convertimur, unde caelum surgit; non tamquam ibi habitet Deus, quasi ceteras mundi partes deseruerit qui ubique praesens est, non locorum spatiis, sed maiestatis potentia; sed ut admoneatur animus ad naturam excellentiorem se convertere, id est ad Deum...

In English, courtesy of CCEL.

... when we stand at prayer, we turn to the east, whence heaven rises: not as if God also were dwelling there, in the sense that He who is everywhere present, not as occupying space, but by the power of His majesty, had forsaken the other parts of the world; but in order that the mind may be admonished to turn to a more excellent nature, that is to God...