The first Christians to be considered as saints were martyrs, that is to say Christians who died during the persecutions because of their faith. From the second half of the 3rd century, when many communities were persecuted, the bodies of these Christians who died for their faith and who were buried outside the city walls attracted the piety of Christians. In the 4th century, the legalization of Christian worship gave a new dimension to it. The Emperor Julian (331-363), who was hostile to the new religion and who, like the Elders, had an aversion for the dead, wrote in his pamphlet Against the Galileans: “You keep adding many new bodies to those of the past. You have filled the world with tombs and graves.”
During the same period, because Constantine favoured places associated with the life of Jesus, Christians started to look for places where Christ could have gone to, where they set up buildings for worship and buildings to accommodate visitors. The first relics (from the Latin word reliquiae, meaning "remains") were the physical remains (bones or objects) related to Christ. This was also the time when some pilgrims disseminated their first testimonies on the routes they had taken. All this contributed to the Christianization of the land.
The two largest pilgrimages of the time were in Palestine and in Rome. Other pilgrimages quickly followed, in the wake of new saints now considered as martyrs. With Christianity being recognized as a religion, the saints were not only martyrs, but also people considered to be worth of sainthood because of the life they lived, because of their qualities as human beings, and because of the miracles linked to their death. The bishop of the place could then confirm the vox populi and “formalize” the local worship.
Some bodies became bodies of saints, as the result of an invention, with no more aversion towards dead bodies and no more respect of the ancient law “Outside the walls”. The relics were brought to the heart of cities, with the development of a full architecture dedicated to them, and with the birth of a new devotion, because of the benefits they were supposed to provide to the faithful. Throughout the first millennium and the beginning of the second millennium, the number of saints grew, as well as the number of relics. The relics were the subject of exchanges, transfers (translations) and sometimes thefts, all of them recounted in the scriptures or in the iconography.
Gradually, the Western Church, with its new organization and a better management of society, set up a centralized procedure for a person to be sanctified. In 1215, the Lateran Council banned all veneration of relics that had not first been approved by the Pope. In 1234, Pope Gregory IX took control of the right to proceed with canonizations by creating a canonization procedure (after a thorough investigation). However, until the 16th century, the approval of the bishop was most of the time sufficient to erect the local cult of a saint.
In the Orthodox Church, popular veneration preceded the formal act of canonization, that came first from Constantinople and then from the Metropolitan, with an increasingly centralized process as well. In Russia, in the 20th century, the Patriarch of Moscow and the National Synod of the Russian Church became the supreme instances for the canonization of national saints. Local bishops played a role only for local saints. An ecumenical council was necessary for ecumenical saints.
Thus, different saints were venerated depending on the countries and the confessions. The cult of relics and their increasing number regularly aroused criticism, particularly from the Protestant Reformers. Calvin wrote a full book about them, named A Treatise on Relics, in 1543. He condemned the trafficking of relics as well as the multiplication of the pieces of the Cross, while using a satirical tone to develop a criticism already made in former times. His book became a best-seller. At the same time, the Council of Trent reaffirmed the worship of relics to counter the criticisms of Protestants.
This cult of relics is not specifically Christian, as shown in pilgrimages created by other religions, for example in Mecca for Islam, or in Hinduism.