There is a mystery to stained glass. When one views it during the day from outside, it appears to be nothing but darkness. There is no beauty, no riot of color, no apparent reason for its existence. But step inside, and what was darkness becomes a thing of beauty and meaning. Mysteries of the faith are brought to life through the artisan's craft. Things are seen that never could have been imagined when standing outside.
This makes for an apt illustration of the Church itself. To remain outside is to be cheated of so much of the beauty of what Christ has done for us, and to miss the fullness of the truth He teaches. Just as natural light, when filtered through stained glass, becomes a thing of immense beauty, so when the Light of Christ is perceived through that "window" of His own creation -- the Church -- we come to know things that only the angels could have imagined.