History of the Christian Altar

In contrast to the Jewish practice of building altars of several stones,^[1] the earliest Christian altars were of wood and shaped like ordinary house tables, a practice that continued until the Middle Ages. However, a preference for more durable materials led to church enactments in the West against wooden altars, but not in the East. The earliest stone altars were the tombs of martyrs, over which Mass was sometimes offered, either on a stone slab enclosing the tomb or on a structure placed above it. When the first custom-built Christian basilicas were built, the altar of the church was placed directly above the tomb of a martyr, as in the case of St. Peter's Basilica and the Basilica of Saint Paul Outside the Walls.^[2]

References

1. For example, Elijah built his altar of twelve stones: "He took twelve stones, for the number of tribes of the sons of Jacob, to whom the LORD had said, 'Your name shall be Israel.' He built an altar in honor of the LORD with the stones, and made a trench around the altar large enough for two seahs of grain" (1 Kings 18:31-33).
2. Maurice Hassett, "History of the Christian Altar" in Catholic Encyclopedia 1907 (with illustrations)