The Prophet says, that he was set by God as a watchtower, which was also fortified, that he might observe the wickedness of the people. In order to gain more authority for his prophecy, he introduces God as the speaker. He had spoken hitherto in his own person; but now God himself comes forth, and says, I have made thee a citadel. Jerome renders the last word "probation." The verb vchn, becken, means to prove; and Jeremiah uses the verb in this verse, "that thou mayest prove their way." But as the word mvtsr, mebezar, "fortress, "follows, we cannot take the word here otherwise than as meaning a citadel or rampart. I therefore have no doubt but that a citadel for watching is what is meant; as though God had said, that his Prophet was like a watchtower, from which might be seen at one glance whatever was done far and wide: for we cannot see far from a plain, but they who are located high can see to a great distance. But the word fortress is also added: for it behooved Jeremiah to watch without fear, and not to be exposed to the threats, calumnies, or clamors of the people. Jeremiah intimates that two things are required in God's servants, even knowledge and undaunted courage; for it was not enough for the prophets to see clearly what was needful, except they were firmly prepared to discharge their office. Both these things seem to be included, when he says, that he was set as a watchtower, and also as a fortress Why was he thus set? That thou mayest know, he says, and prove their way Let us now see what was the intention of this. The Prophet no doubt here claims power and credit to himself, that he might not only freely but authoritatively reprove the people: for objections, we know, were ever in their mouths, that they might be at liberty to despise the Prophet's teaching, as though it did not proceed from God. This then was the reason why God here declares that Jeremiah was like a citadel, and that a fortified one; he was made so, that he might observe and know the way of the people. Hence it followed, that however obstinately they might defend themselves, it availed them nothing; for Jeremiah was endued with the highest authority, even that which was divine, in order to perform his office of a judge in condemning them: for it immediately follows --

Render it:
I have set thee among My people as a prover of ore,
And thou shalt know and try their way.
They are all of them rebels of rebels (i. e., utter rebels):
Slander-walkers, were copper and iron,
Corrupters all of them.
The bellows glow: from their fire lead only!
In vain hath the smelter smelted,
And the wicked are not separated.
Refuse-silver have men called them:
For Yahweh hath refused them.
The intermixture throughout of moral words and metallurgical terms is remarkable.

I have set thee for a tower and a fortress - Dr. Blayney translates, I have appointed thee to make an assay among my people. The words refer to the office of an assayer of silver and gold; and the manner of assaying here intended is by the cupel, a flat broad iron ring filled with the ashes of burnt bones. To separate the alloy from the silver they add a portion of lead; and when all is fused together, and brought into a state of ebullition, the cupel absorbs the lead, and with it the dross or alloy, and the silver is left pure and motionless on the top of the cupel. The people are here represented under the notion of alloyed silver. They are full of impurities; and they are put into the hands of the prophet, the assayer, to be purified. The bellows are placed, the fire is lighted up, but all to no purpose: so intensely commixed is the alloy with the silver, that it can not be separated. The nozzle of the bellows is even melted with the intensity of the fire used to effect the refinement; and the lead is carried off by the action of the heat; and the assayer melteth in vain, for the alloy still continues in union with the metal. The assayer gives up the process, - will not institute one more expensive or tedious - pronounces the mass unfit to be coined, and denominates it reprobate silver, Jeremiah 6:30. Thus, the evil habits and dispositions of the Israelites were so ingrained that they would not yield to either the ordinary or extraordinary means of salvation. God pronounces them reprobate silver, - not sterling, - full of alloy; - having neither the image nor the superscription of the Great King either on their hearts or on their conduct. Thus he gave them up as incorrigible, and their adversaries prevailed against them. This should be a warning to other nations, and indeed to the Christian Church; for if God did not spare the natural branches, neither will he spare these.

I have set (u) thee [for] a tower [and] a fortress among my people, that thou mayest know and try their way.
(u) Meaning, Jeremiah, whom God had appointed to try out the godly from the wicked, as a founder does the pure metal from the dross.

I have set thee for a tower,.... Or "in" one (d); in a watch tower, to look about and observe the actions of the people, their sins and transgressions, and reprove them for them; as well as to descry the enemy, and give notice of danger; see Habakkuk 2:1 or, "for a trier"; since the word used comes from one which signifies to "try" metals, as gold and silver; and the rather this may be thought to be the meaning here, since the verb is made use of in this sense in the text; and the metaphor is carried on in the following words; though the word is used for towers in Isaiah 23:13 and may well enough be understood of a watchtower, agreeably with the office of the prophet; who is here addressed as a watchman, and was one to the house of Israel: and as the faithful discharge of his work required courage, as well as diligence and faithfulness, it follows, and
for a fortress among my people; not to defend them, but himself against them; or he was to consider himself as so under the divine protection, that he was as a fortress or strong tower, impregnable, and not to be dismayed and terrified with their calumnies and threatenings; see Jeremiah 1:18,
that thou mayest know and try their way; their course and manner of life, whether good or bad; which he would be able to do, being in his watch tower, and in the discharge of his duty; for the ministry of a good man is as a touchstone, by which the principles and practices of men are tried and known; for if it is heard and attended to with pleasure, it shows that the principles and practices of men are good; but if despised and rejected, the contrary is evident, see 1-John 4:5.
(d) "in exploratoria specula", Junius & Tremellius.

tower . . . fortress-- (Jeremiah 1:18), rather, "an assayer (and) explorer." By a metaphor from metallurgy in Jeremiah 6:27-30, Jehovah, in conclusion, confirms the prophet in his office, and the latter sums up the description of the reprobate people on whom he had to work. The Hebrew for "assayer" (English Version, "tower") is from a root "to try" metals. "Explorer" (English Version, "fortress") is from an Arabic root, "keen-sighted"; or a Hebrew root, "cutting," that is, separating the metal from the dross [EWALD]. GESENIUS translates as English Version, "fortress," which does not accord with the previous "assayer."

I have set thee - Here God speaks by way of encouragement to the prophet, and tells him, he had made him a fortified tower, that he might be safe, notwithstanding all the attempts against him. And try - As refiners do metals; hereby be is encouraged to reprove them more freely, God will give him prudence to see what is amiss, and undauntedness to oppose it.