"Congregation." Not the building, therefore; because if Mass was offered up in an open field, with the people kneeling about, it would still be the church of that place. The buildings that we use for churches might have been used for anything else--a public hall, theater, or school, for example; but when these buildings we call churches are blessed or consecrated, they become holy. They are holy also because the Gospel is preached in them, the Sacraments are administered in them, and the Holy Sacrifice of the Mass is offered in them. But they are holy especially because Our Lord dwells in them in the tabernacle, where He lives and sees and hears just as truly as He did when He was man upon earth. In the early ages the Christians had no churches--they met secretly in private houses. Later, when the cruel pagan emperors began to persecute and put to death the Christians, they made large tunnels under ground and in these places they heard Mass and received the Sacraments. These underground churches were called the catacombs, and some of them may still be seen at Rome. In these catacombs, too, the Christians buried their dead, especially the bodies of the holy martyrs. On their tombs--generally of stone--Mass was celebrated. In every altar the table, or flat part on which the priest celebrates Mass, should be of stone; but if the altar is made of wood, then at least the part just in front of the tabernacle must be of stone and large enough to hold say two chalices--that is, about ten or twelve inches square. In this stone are placed some relics of the holy martyrs. A piece is cut out of the stone and the relic placed in the opening. Then the bishop puts the little piece of stone back into its place over the relic, seals the opening, blesses the stone, and gives it to the Church. This is called the altar stone. You cannot see it because it is covered with the altar cloth; but unless it is in the altar the priest cannot say Mass. This stone reminds us of the stone tombs of the saints upon which Mass was celebrated.