I. They built an altar, and offered sacrifice to God (v. 30, 31), in token of their dedication of themselves to God, as living sacrifices to his honour, in and by a Mediator, who is the altar that sanctifies this gift. This altar was erected on Mount Ebal, the mount on which the curse was put (Deut. xi. 29), to signify that there, where by the law we had reason to expect a curse, by Christ's sacrifice of himself for us and his mediation we have peace with God; he has redeemed us from the curse of the law by being made a curse for us, Gal. iii. 13. Even where it was said, by the curse, You are not my people, there it is said, through Christ the altar, You are the children of the living God, Hos. i. 10. The curses pronounced on Mount Ebal would immediately have been executed if atonement had not been made by sacrifice. By the sacrifices offered on this altar they did likewise give God the glory of the victories they had already obtained, as Exod. xvii. 15. Now that they had had the comfort of them, in the spoils of Ai, it was fit that God should have the praise of them. And they also implored his favour for their future success; for supplications as well as thanksgivings were intended in their peace-offerings. The way to prosper in all that we put our hand to is to take God along with us, and in all our ways to acknowledge him by prayer, praise, and dependence. The altar they built was of rough unhewn stone, according to the law (Exod. xx. 25), for that which is most plain and natural, and least artful and affected, in the worship of God, he is best pleased with. Man's device can add no beauty to God's institutions.