The practice is as old as the tombs of the martyrs and was defended by the Council of Trent in its twenty-fifth session, which taught that the bodies of the martyrs and of others now living with Christ are to be venerated by the faithful. It is expressed liturgically in the ancient custom of placing relics of martyrs beneath a fixed altar. Relics are commonly classed as first class (the body of a saint), second class (something the saint used or wore), and third class (an object touched to the first or second). The Congregation for the Causes of Saints regulates their authentication and conservation in its 2017 Instruction, and canon law absolutely forbids selling them or transferring them without the permission of the Holy See. The honour given is veneration of the saint and worship of God who is wonderful in his saints — never adoration of the object.

Venerate a relic as the Church does: approach quietly, kneel or bow, and kiss or touch the reliquary. Then pray — not to the relic but to God, asking through the intercession of the saint whose relic it is, and read something of that saint's life or writings so that the veneration leads to imitation. Never buy or sell a relic; if one comes into your hands, entrust it to a parish or a religious house where it can be kept and honoured properly, with its authentication intact.