On a spring Sunday in the late twelfth century, on the occasion of the Feast of the Annunciation, a preacher at the Augustinian abbey of St-Victor de Paris concluded his sermon by speaking about stained glass. The biblical theme of the sermon is Luke 1:31: ‘Behold, you shall conceive and bear a son.’ The anonymous preacher had been urging his fellow canons to emulate the Virgin Mary, so that they too might become worthy of receiving Christ. We cannot know exactly which window, if any, he was looking at or considering. The abbey church was destroyed in the eighteenth century, and we know little about its decoration. A typical Annunciation scene from the period, however, would offer a range of symbols and images that could serve to model virtue. Mary’s head is generally bowed, displaying her obedience and humility. She is veiled, indicating her modesty. She might hold a variety of iconographical symbols: a book, signifying the Incarnation of the Word, or Mary's wisdom, or her devotion; a water jar, sign of purity; a lily, symbol of virginity and/or the resurrection; a rose without thorns, to show her sinlessness. She tends to be dressed in blue, the color of fidelity; or green, conveying regeneration; and/or in red, which signals both suffering and love. However, the preacher makes no reference to any of these visual devices; he says nothing about Mary's expression or gestures, posture or attire, or any object symbolic of faith or virtue. Indeed, he does not refer to the iconography or narrative of stained glass at all. Instead, he discusses its raw materials:

For it must be known that the carnal soul cannot receive Christ, just as a wooden window [meaning presumably a window with a wooden shutter] cannot receive a sunbeam. And just as a glass window does receive a sunbeam, and the ray itself enters and penetrates it, so the spiritual soul, purified of filth and cleansed from sins, conceives Christ.

This analogy echoes, and likely was inspired by, a well-known explanation of the virginal conception. Often attributed to Bernard of Clairvaux, it noted that the seed of God could pass through Mary without piercing her, just as a ray of light can pass through a glass window without breaking it. But the preacher does not stop there.