Exodus 27:8

Hollow with boards shalt thou make it; as it was showed thee in the mount, so shall they make it. It is probable that the hollow space inside the altar was always filled with earth, Exodus 20:24, the place for the fire being in the center of this square and far enough from the wooden walls to obviate the danger of setting these afire. This altar was always in plain sight before all the children of Israel, reminding them of the fact that an expiation of sins was needed. The altar of the Christians is the cross of Christ, on which He bare our sins, that we, being dead to sins, should live unto righteousness, 1 Peter 2:24; Hebrews 13:10.