Deuteronomy 4:20

“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.”

“4:20 An iron-smelting furnace was used to refine metal ores and separate the pure metal from the dross. Israel’s trials in Egypt made the Israelites more spiritually and morally pure. • special possession (literally people of inheritance): Israel itself was God’s inheritance. This concept is confirmed by the parallel phrase describing Israel as his very own people. As such, the Israelites were now fit to enter, conquer, and occupy the land (cp. 1 Pet 2:9-12).”

“the iron: Kg1 8:51; Jer 11:4 a people: Deu 9:26, Deu 9:29, Deu 32:9; Exo 19:5, Exo 19:6; Psa 28:9, Psa 33:12, Psa 135:4; Isa 63:17, Isa 63:18; Eph 1:18; Tit 2:14; Pe1 2:9 Deuteronomy 4:21”

“The Israelites were not to imitate the heathen in this respect, because Jehovah, who brought them out of the iron furnace of Egypt, had taken them (לקח) to Himself, i.e., had drawn them out or separated them from the rest of the nations, to be a people of inheritance. They were therefore not to seek God and pray to Him in any kind of creature, but to worship Him without image and form, in a manner corresponding to His own nature, which had been manifested in no form, and therefore could not be imitated. בּרזל כּוּר, an iron furnace, or furnace for smelting iron, is a significant figure descriptive of the terrible sufferings endured by Israel in Egypt. נחלה עם (a people of inheritance) is…”