In this post, a few thoughts on why medieval churches cover themselves in stories.
There are three times: a present of things past, a present of things present, and a present of things to come. For these three times somehow exist in the soul, for otherwise I could not see them. The time present of things past is memory; the time present of things present is direct experience; the time present of things future is expectation.
---Augustine, Confessions, chapter 20
Baptistery of San Giovanni, Florence
This is the baptistery of the medieval elite of Florence, including Dante Alighieri's family. The mosaic ceiling not only holds Christ Pantocrator, but also heaven, hell, the Creation, the life of Christ, the life of Mary, and a host of angels. There is an oculis/lantern in the ceiling, so that natural light passes into the building, illuminates the golden mosaics, and transforms into divine light. Under this dome, infants are presented by their families, and initiated into both their spiritual and public life. Their story officially begins with their baptized soul, and they become the newest participant in the story of the Church, from the Creation to the End of Days, and the triumph of Christianity in eternity.
I've spent decades studying and photographing religious buildings, especially medieval ones, and I've moved through several thoughts on the purposes of the art on the interior and exterior. Certainly the stories painted, carved, or embedded into the walls were in-part meant to instruct. A long-standing argument has been that the instruction was for the illiterate, who could not read the holy stories themselves. However, religious art abounds in monasteries, scriptoriums, palaces--places where literate people dwelt. Moreover, through much of medieval Christianity, the Church preferred for the faithful to learn the stories of their creed through sermons and catechism, not private study. So, the idea of church art as a "poor man's Bible" doesn't answer the question church art's purpose fully.
Baptistery of Parma
The baptistery in Parma was built a century later than the one in Florence. Its paintings include all the themes of the one in Florence, plus portraits of saints, prophets, apostles, and tableaux of local miracles. The paintings cover the walls as well as the ceiling. There are also zodiacal creatures and labors of the months in sculptures in niches. There's scarcely an inch not ornamented, and stories occasionally repeat. The stories and figures attend the ritual of baptism, reminding the participants of the beliefs and expectations they are supporting, but also reminding the participants that they are, in turn, supported by their religion.
Rose Window, Chartres Cathedral
The artwork in churches is often quite high-up, and difficult to discern in detail. Consider the rose windows at Notre Dame at Chartres. They are exquisitely detailed, but it would take a person with vision much greater than average to discern that detail, or make out all of the figures. Even with a zoom lens, it takes guidance for me to figure out which stories are being told. Churches which were pilgrimage sites had monks or priests who would explain the stories in the walls to visitors, so that would help. The art then, would be an occasion to educate and encourage worshippers. But leaders who created the program of narratives in their churches also used the art to give God evidence of devotion, as an offering. God could see through those windows, and could see ornament not visible to people below.
Temple of Hercules Victorious, Rome
Pre-Christian, European polytheism tended to rituals and worship outside of a temple. In some instances, it was because the natural space was itself holy, and the temple was added to the site to facilitate and focus ceremonies. The stories for temples were on the exterior, with an idol representing the god or goddess within, along with a repository for votive offerings of great value. This temple in Rome was placed in the old cattle market square. It was built in the 2nd century B.C., and only survived the forced conversions to Christianity by being made into a small church. The original roof may have been a marble dome. Early Christian worship, for multiple reasons (not the least of which was fear of persecution), evolved from worshipping in private spaces and literally underground--in Roman catacombs. The earliest liturgy was not processional, but modeled on eating together, and praying or singing together in rooms in houses. Pagan worship also featured people eating together--from the cooked meats of their animal sacrifice, but also had processions, competitions (like the Olympics), and initiation rites for different stages of life. Christian worship developed many of these features, but retained a determination to keep their mysteries and communities in contained spaces.
Revelation in Stained Glass, Sainte Chapelle
Early Christianity's interior spaces--especially those related to martyrdom, evoked death and rebirth in an enclosed place: from the tomb, the dead Christ disappears; in his empty tomb the good news is revealed to the three women who had come to grieve. When churches could be built in the public realm, they became both palatial (church triumphant) and protective (church militant). Church interiors and the rituals within (which did develop into processions, quite different from the sitting that is featured in the last couple of centuries) were meant to create heaven on earth. So, lavish, costly art was deemed appropriate.
Where early Roman Christians who died for attesting their faith were buried, secret worship was performed over their sarcophagi in the catacombs. Altars in most later medieval Christian churches were either sarcophagi for saints, or were tables placed directly over where those bodies or relics lay in the crypt below.
There is also scriptural precedent for the placement of martyrs' bodies under altars, coming from Revelation 6:9-11:
"When he opened the fifth seal, I saw under the altar the souls of those who had been slaughtered for the word of God and for the testimony they had given; they cried out with a loud voice, “Sovereign Lord, holy and true, how long will it be before you judge and avenge our blood on the inhabitants of the earth?” They were each given a white robe and told to rest a little longer, until the number would be complete both of their fellow servants and of their brothers and sisters, who were soon to be killed as they themselves had been killed."