St. Felix I, who governed the Church from 268 to 273, was buried after his martyrdom in the “crypt of the popes” at the catacomb of St. Callixtus.  Pope St. Felix commanded that Masses be offered over the tombs of martyrs.  This union of the martyrs with the sacrifice of the Mass shows that their sufferings, united with the sacrifice of Christ, leads to a glorious resurrection with Him.  This custom still exists today because Mass must be offered on an altar which contains a stone containing the relics of the martyrs.