The cult of relics developed in the very earliest years of the Christian religion. See Peter Brown, The Cult of the Saints in the Early Church (Princeton, 1985). The Christian attitude, in distinction from the pagan treatment of the dead, characterized those who “slept in Christ” as members of an extended Christian community.  Burial places were not “outside the walls” of cities, as proscribed in Roman law, but clustered around, in, and under places of worship.  Places of worship grew up over the sites of significant graves.  In the fourth century, the Emperor Constantine built the basilica of St. Peter over a cemetery believed to contain the grave of the first pope.  Constantine and his mother St. Helena built the Church of the Holy Sepulchre to enshrine both the place of Christ’s death and his tomb. 

The demand to be close to the tangible remains of heroic Christians, the great confessors and martyrs, especially in the founding of new churches, encouraged the partition of bodies to allow the sacred “aura” that facilitated God’s grace to be shared among a growing community.  Churches were founded with relics as their essential “talisman.”  In 396 the French city of Rouen celebrated the arrival of relics from Rome.  Victricius, speaking for the Christian community, stated: “Give me these temples of saints. . . If a light touch of the hem of the savior’s garment could cure (reference to Christ healing the woman with the issue of blood when she touched his robe, described in Luke 8:43-48), then there is no doubt that these dwelling places of martyrdom (the relics) carried in our arms, will cure us. . . Let us draw down on us the favor of the saints. . . Their dwelling is on high, but let us evoke them as our guests.” (see Christianity and Paganism, 350-750, ed. J. N. Hillgarth (Philadelphia, 1986), 23 

Throughout the entire Middle Ages, the possession of relics of important saints made sites popular. Veneration even included significant displacement to visits theses relics.  Margery Kempe’s visits invariable mention relics, even locally, such as the tomb of St. William of Norwich.  They encouraged her wide travels to the Holy Land and the Holy Sepulchre, Santiago of Compostela, and the tomb of St. James the Great, or Aachen, among many others.  Christians did not believe that the souls of the saints dwelt in their relics (bones, clothes they wore, or elements of their martyrdom, such as the stones used to kill Stephen), but that these things would act as conduits to grace.  They would link the revered intercessor, the favored saint in the face of God, to his or her faithful on earth.  Evocation of the saint was not only a way of petitioning favor but was a means to emulate the saint’s virtues of courage, generosity, or resistance to temptation.  For example, Margery Kempe continually evokes Mary Magdalene’s repentance for her sexual exercise as a model for her own desire to achieve spiritual virginity. 

Theft of the body of St. Martin.  Martin, the patron saint of Tours died at Candes.  His followers contrived to steal the body.  The image shows them passing it out a window for transportation on the Loire river to the city of Tours where a great pilgrimage church was built.  This type of activity was called a “pious theft” and its practitioners (as for the removal the body of St. Benedict from Montecassino in Italy to Fleury on the Loire, France) was an affirmation of God’s judgment that the original owners were not worthy of the honor. 

Martin’s Body removed, detail of the window of St. Martin, Tours Cathedral, c. 1260. © Raguin/MMK
The Pilgrim's Guide to Saint James of Compostela written in the 12th century describes the great basilica, now destroyed, at Tours. 

Then on this same road along the Loire, one should visit the worthy body of the blessed Martin, bishop and confessor.  In truth, he is said to be the noble resuscitator of three dead persons, and he is said to have restored to desired health, lepers, the possessed, those who had gone astray in their wits, madmen, and those possessed by devils, and other sick people.  For truly the tomb in which rests his most sacred mortal clay near the city of Tours gleams with a profusion of silver and gold and precious stones, and it shines forth with frequent miracles. Above it an immense and venerable basilica of admirable workmanship, similar to the church of the blessed James [of Compostela], was built in his honor; to it the sick come and are cured, the possessed are delivered, the blind are given sight, the lame are raised up and all kinds of illnesses are cured and total consolation is given to all who ask.  That is why his splendid renown is spread everywhere by just praise, to the glory of Christ.  His festival is celebrated on the third day before the Ides of November [11 November]
Annie Shaver-Crandell and Paula Gerson, The Pilgrim's Guide to Santiago de Compostela: A Gazetteer with 580 Illustrations (London: Harvey Miller Publishers, 1995), 81. 

Canterbury Cathedral, detail of window in Trinity Chapel showing the Shrine of St. Thomas Becket © Raguin/MMK.  The shrine was designed by Elias of Durham sometime between 1205 to1216 (destroyed by Henry VIII).  Faced with gold and decorated with jewels, it stood on an elevated base in the middle of the Trinity Chapel, at the east end of the church.  Pilgrims walked around the shrine, framed by windows of the miracles of Becket (as Chaucer says), "the holy blissful martyr, who helps (pilgrims) when they are sick."

St. William's Shrine depicted in the window of the saint, York Minster.  The large window constructed around 1415 depicts both the virtues of the saint and the many pilgrims coming to be cured at his shrine.

A woman afflicted with dropsy prays at St William's shrine.

A man east mortar from St. William's shrine in the hope of a cure.

A Mother and Child give thanks at St. Williams' shrine for a recovery.

A man offers a wax model of a leg to St. William's shrine.  This is commonly called an "ex voto."  Such offerings of images of body parts have been documented in Roman practice, for example at Bath in England. 

A prisoner in stocks prays to St. William.

A prisoner offers his fetters as an "ex voto" at St. William's shrine. 
 

For an extensive catalogue of the reliquaries in the treasury of the Abbey of St. Denis, see Alison Stones, Images of Medieval Art and Architecture.