Light is truly the inspiration for stained glass in both the physical and literal feelings. During the Gothic Era from about 1150 - 1500 A.D. there was no electricity so alternate ways of lighting Cathedrals was necessary. Immense and elegant stained glass windows were created and intended to provide physical light by allowing in much needed sunlight, but they were also meant to provide spiritual light. The King James Version of the Bible in John 8:12 says "Then spake Jesus again unto them, saying, I am the light around the world: he that followeth me shall not walk in the night, but shall have the light of life." Jesus was the inspiration for and his life, gruesome death and subsequent resurrection were the subject of nearly all of stained glass windows made during that period in history.
Grand and magnificent cathedrals carved up gigantic stones and supported by immense pillars and flying buttresses reaching up towards the Heavens in worship of God and His son were crowned with pleasurable jewels known as stained glass. In a few Cathedrals, stained glass planks covered entire walls and the nourishing pillars go almost entirely unnoticed. The pillars alone couldn't have supported the weight of the structure, which is the aim of the flying buttresses (they braced the structure from the outside).
The skill of making stained glass has been poetically brought up as "painting with light" taking the analogy even further. This term was coined owing to the belief that as opposed to reflecting light off of it, a stained glass window pane allows light to be transmitted through it. It is a unique partnership, as neither the light nor the window is as magnificent without the other.
Abbot Suger of the Cathedral at St. Denis in France was among the first one to employ the Gothic type of architecture in a shot to glorify God and Jesus Christ. The next quote is learned a writing of Suger, included as a piece of a transcription on the thresholds to the Cathedral. That gives insight into his motivations for using large amounts of stained glass and the relationship of the physical light to the spiritual, "...The noble work is bright, but, being nobly bright, the effort should brighten the minds, allowing them to travel through the lights to the true light, where Christ is the true threshold."
He later gave a detailed explanation as to what the aim of the exemplary works of stained glass window art were in the church; "Thus occasionally when, as a consequence of my relish the beauty of the home of God, the multicolor loveliness of the gems has called me faraway from outside cares, and worthy meditation, transporting me from material to immaterial things, has persuaded me to look at the number of holy virtues, then I seem to observe myself existing on some level, as it were, beyond our earthly one, neither completely in the slime of earth nor completely in the purity of heaven.
By the gift of God I am in a position to be transported in an anagogical manner from this inferior level to that superior one." Walking into a mighty cathedral like St. Denis, even today, one can feel the influence this passion for light had on the growth of the art of stained glass making and the worship of God during the Gothic Era.