Holy relics can be found in many different religions, whether Christian, pagan, Hindu, or Buddhist. A relic is defined as something directly associated with a revered saint, teacher, ancestor, or some miraculous manifestation of deity in the material world. A relic and a reliquary are two types of sacramental tools. A reliquary is a container or box for a relic, allowing it to be displayed to the public and thus transfer its powers to magical and liturgical works. There is another definition of a relic: anything that would be a miraculous manifestation of a deity in the material world. In Christianity, there are countless numbers of relics, most of them being associated with Jesus Christ.

In the early years of Christianity, it was forbidden to move and share the remains of saints and martyrs, and they were protected so that their final resting places, such as catacombs and cemeteries, remained intact. The earliest recorded Christian relics, specifically the bones and ashes of martyrs, date from the second century. Two hundred years later, St. Ambrose and others set out to dig up the corpses and honor them. At that time, it was considered that every holy body could be dismembered and divided among the faithful to worship it. The remains of corpses sometimes came into someone else's hands as a gift, and sometimes they were stolen. At one point, the demand for holy bodies was such that bodyguards were assigned to dying saints to prevent their bodies from being dismembered after death.

The veneration of the relics of saints was so common among the faithful because they believed saints in Heaven interceded for believers on Earth. As a result, numerous healings and miracles were attributed to the relics, and many stories and myths about miracles arose from these beliefs. By word of mouth, the stories spread among the faithful, and finally, they were recorded in books of hagiographies such as The Golden Legend by Jacobus de Voragine. Inevitably, the demand for relics grew with the popularization of stories about their miracles, so a rich trade in relics developed indirectly.

The use of relics, perhaps the greatest superstition, reveals the deception and inconsistency that Christians have been subjected to for centuries. Parts of the True Cross were among the most worshipped relics, but there were so many of them scattered across Europe and other parts of the world that Calvin once joked that if all the pieces were collected, they would form a shipload, despite the fact the cross was only large enough that one person could carry it. To get around this conundrum, figures like St. Paulinus spoke of the reintegration of the cross—in other words, claiming that the cross never diminished, no matter how many pieces were broken off from it.

Arma Christi is a collective name for objects, devices, and symbols related to the Passion of Christ. They are also called tools of Christ or instruments of suffering. They have often been depicted in art, iconography, Christian symbolism, and piety, and they had a particularly high value in the Middle Ages. Their purpose was to arouse a feeling of gratitude to Christ for the gift of redemption.

Legends aside, the real problem arises when historians eliminate the mythical elements and try to identify how and when these relics appeared for the first time in history, and from there, follow them from generation to generation and from church to church, through the hands of kings and popes. It is a long and complicated history, from believers of Christianity to its enemies, and historians have to try to determine how the relics were lost and recovered, how they were scattered all over the world (and only sometimes reunited again), how they were stolen by invading armies and used as impressive forms of propaganda; and even how they were lost, recovered, and deposited in the places where they are venerated today.