Jamieson, Fausset and Brown on Deuteronomy 4:20

20. But the Lord hath taken you, and brought you forth out of the iron furnace--that is, furnace for smelting iron. A furnace of this kind is round, sometimes thirty feet deep, and requiring the highest intensity of heat. Such is the tremendous image chosen to represent the bondage and affliction of the Israelites [Rosenmuller].

to be unto him a people of inheritance--His peculiar possession from age to age; and therefore for you to abandon His worship for that of idols, especially the gross and debasing system of idolatry that prevails among the Egyptians, would be the greatest folly--the blackest ingratitude.