i.e. The furnace wherein iron and other metals are melted, to which Egypt is fitly compared, not only for the torment and misery which they there endured, but also because they were thoroughly tried and purged thereby, as metals are by the fire. A people of inheritance; his peculiar possession from generation to generation. See Exodus 19:5 Deuteronomy 7:6 Titus 2:14. And therefore for you to forsake God, and worship idols, will be not only wickedness and madness, but most abominable ingratitude.
Related Commentaries of Deuteronomy 4:20
Deuteronomy 4:20
20 But the LORD hath taken you, and brought you forth out of the iron furnace, even out of Egypt, to be unto him a people of inheritance, as ye are this day.