B-16's "Church Windows"--the Source

The valuable Fr. Z. knows the work and quotes for us the passages (set in context also worth reading.) This is the reference Benedict XVI made in his sermon at St Patrick's.

These were the painted windows; and as often as he gazed at them the sculptor blessed the medieval time, and its gorgeous contrivances of splendor; for surely the skill of man has never accomplished, nor his mind imagined, any other beauty or glory worthy to be compared with these.

It is the special excellence of pictured glass, that the light, which falls merely on the outside of other pictures, is here interfused throughout the work; it illuminates the design, and invests it with a living radiance; and in requital the unfading colors transmute the common daylight into a miracle of richness and glory in its passage through the heavenly substance of the blessed and angelic shapes which throng the high-arched window.

And, as the Pope mentioned, it's very different when looking from the outside.

The friends left the church, and looking up, from the exterior, at the window which they had just been contemplating within, nothing was visible but the merest outline of dusky shapes. Neither the individual likeness of saint, angel, nor Saviour and far less the combined scheme and purport of the picture, could anywise be made out. That miracle of radiant art, thus viewed, was nothing better than an incomprehensible obscurity, without a gleam of beauty to induce the beholder to attempt unravelling it.

"All this," thought the sculptor, "is a most forcible emblem of the different aspect of religious truth and sacred story, as viewed from the warm interior of belief, or from its cold and dreary outside. Christian faith is a grand cathedral, with divinely pictured windows. Standing without, you see no glory, nor can possibly imagine any; standing within, every ray of light reveals a harmony of unspeakable splendors."

Those passages from Nathaniel Hawthorne's book The Marble Faun, which describes the American author's trip through Italy.

There's a lot more of the pertinent passage at the link, and worth reading--particularly the brief passage on 'darkness.'