Which I commanded your fathers in the day that I brought them forth out of the land of Egypt, from the iron furnace, saying, Obey my voice, and do them, according to all which I command you: so shall ye be my people, and I will be your God:
In the day - i:e., when. The Sinaic covenant was some time after the exodus, but the two events are so connected as to be viewed as one.
Iron furnace - (Deuteronomy 4:20; 1 Kings 8:51). "Furnace" expresses the searching ordeal; "iron," the long duration of it. The furnace was of earth, not of iron (Psalms 12:6); a furnace in heat and duration enough to melt even iron. God's deliverance of them from such an ordeal of hard bondage in Egypt aggravates their present guilt.
Do them - namely, the words of the covenant (Jeremiah 11:3).
So shall ye be my people - (Leviticus 26:3; Leviticus 26:12, "If ye walk in my statutes, etc., I will walk among you, and will be your God, and ye shall be my people").