An altar of earth. Among the nations of antiquity altars were indispensable to Divine worship, which everywhere included sacrifice. They were often provided on the spur of the occasion, and were then "constructed of earth, sods, or stones, collected upon the spot."
The patriarchal altars bad probably been of this character, and it was now provided that the same usage should continue: at any rate, elaborate structures of hewn and highly ornamented stone should not be allowed, lest thus idolatry should creep in, the images engraved upon the altars becoming the objects of worship.
Thy burnt offerings and thy peace offerings. The mode in which these are introduced implies that sacrifice was already a long-standing practice. The patriarchal sacrifices are well known (Genesis 8:20; Genesis 12:7; Genesis 22:9; Genesis 35:1).
Jethro had recently offered sacrifice in the camp of Israel (Exodus 18:12). If the Israelites had not sacrificed to God during the sojourn in Egypt, at any rate they had kept up the idea of sacrifice; and it was for the purpose of offering sacrifices that Moses had demanded permission to go with all his nation into the wilderness.
I will come unto thee and I will bless thee. The promise is conditional on the observance of the command. If the altars are rightly constructed, and proper victims offered, then, in all places where he allows the erection of an altar, God will accept the sacrifices offered upon it and bless the worshippers.