Thomas Aquinas Commentary Jeremiah 6:27-30

"I have made thee a trier [and] a fortress among my people; that thou mayest know and try their way. They are all grievous revolters, going about with slanders; they are brass and iron: they all of them deal corruptly. The bellows blow fiercely; the lead is consumed of the fire: in vain do they go on refining; for the wicked are not plucked away. Refuse silver shall men them, because Jehovah hath rejected them." — Jeremiah 6:27-30 (ASV)

Here, he shows the people’s inability to be corrected.

1. He first brings up the testimony of the prophet: as an assayer, so that you may learn of their stubbornness as if by experience, while being unable to turn them to what is good. Know well the state of your cattle, and consider your herds (Proverbs 27:23).
2. He then discusses their universal sin: all these princes turn aside. Bronze and iron. It is as if he were saying, “Not only are they unclean, but they are covered in uncleanness,” just as iron and bronze are impurities in silver. Alternatively, iron represents their hardness, and bronze their inability to bear anything, because it is brittle. His bones are like reeds of bronze (Job 40:18). They are corrupt and have become abominable in their desires (Psalms 13:1).
3. He gives an example of their inability to be corrected: the bellows have failed. Silver mixed with lead is first placed in the furnace. Then, while the silversmith works the bellows, the dross is consumed and the silver is improved, but the lead is diminished.Thomas Aquinas is perhaps speaking here of the process of refining galena, a lead ore containing significant quantities of silver. Now, these people were put in the furnace of tribulation and consumed with evil as if with lead, but they were not corrected. In vain, says the word of God, has the founder—that is, God or the preacher—melted them. In fire are gold and silver tried, but acceptable men in the furnace of humiliation .
4. He concludes the argument for their rejection: reprobate silver you shall call them. You have utterly rejected us, you are exceedingly angry with us (Lamentations 5:22).

Collations

It should be noted that sin is called cold (Jeremiah 6:7) because it extinguishes the heat of love. Because wickedness abounded, the love of many will grow cold (Matthew 24:12). Sin is also called cold because it freezes the moisture of love: the north wind has blown cold, and frozen the water to crystal . And it is called cold because it slows the activity of good works: the lazy man refuses to plow because of the cold, therefore in the summer will he beg, but it shall not be given him (Proverbs 20:4).

Also, note that certain things are called ways (Jeremiah 6:16). These include the ways of the commandments: I have run the way of your commandments, for you have broadened my heart (Psalms 118:32); of actions: let us examine our ways, and inquire, let us return to the Lord (Lamentations 3:40); of virtues: I will show you the way of wisdom, and will lead you along the paths of equity (Proverbs 4:11); of teachers: her ways are lovely, and all her paths peace (Proverbs 3:17); and Christ himself: I am the way, the truth and the life (John 14:6).