As part of Abbot Suger's renewal of art in the abbey of Saint Denis in France in the 1140s, he had "a splendid variety of new windows" painted. The windows were more than pretty decoration. For example, the window of the Apostle Paul turning a mill had the effect of "urging us onward from the material to the immaterial". (1) The inclusion of stained glass in Abbot Suger's project, considered by many the first church in the Gothic style, set a precedent for the inclusion of stained glass in churches built in the 12th through 14th centuries.
Because stained glass was both transparent and colored, it was believed to mediate between the heavenly and earthly. Light passing through glass intact was a miraculous property comparable to Christ being conceived and born of the Virgin. Thus stained glass in itself had a heavenly quality.
The didactic function of windows complemented this heavenly aspect because the stories and images also brought the heavenly within reach of the earthly. The images brought the stories of Christ and the saints to vibrant life. To the modern church visitor, these stories often seem unintelligible. This happens for several reasons. First, the stories are not always told in a linear fashion. Although they usually progress from the ground up, they do not always read left to right, often snaking through the images instead. Second, the glass often shows events recorded outside of the biblical scriptures in saint's lives, medieval poems, and theological writings (2). These additions / changes to the stories may not be familiar to modern viewers.
Because of these functions, those who donated windows were seen as attaining a higher spiritual state. The scholar Madeline Caviness noted one incident in which the Bishop of Paris refused to let a group of prostitutes donate a window for the Cathedral because he objected to sinful way in which the money was raised (3).
Modern visitors to the medieval churches of Europe still have the chance to experience stained glass and its heavenly qualities. The light still shines through them, illuminating color and image which can be interpreted by the modern viewer in new ways.
(1) Erwin Panofsky, Abbot Suger. On the Abbey Church of St.-Denis and its Art Treasures (Princeton: Princeton University Press, 1979), 73-75.
(2) Madeline Caviness, "Biblical Stories in Windows: Were they Bibles for the Poor?" in Bernard Levy, ed., The Bible in the Middle Ages: Its Influence on Literature and Art (Binghamton, NY: Medieval & Renaissance Texts & Studies, 1992), 103-147.
(3) Madeline Caviness, Stained Glass Windows (Tornhout, Belgium: Brepols, 1996), 59.