6:27 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.
6:27 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.
6:18-30 God rejects their outward services, as worthless to atone for their sins. Sacrifice and incense were to direct them to a Mediator; but when offered to purchase a license to go on in sin, they provoke God. The sins of God's professing people make them an easy prey to their enemies. They dare not show themselves. Saints may rejoice in hope of God's mercies, though they see them only in the promise: sinners must mourn for fear of God's judgments, though they see them only in the threatenings. They are the worst of revolters, and are all corrupters. Sinners soon become tempters. They are compared to ore supposed to have good metal in it, but which proves all dross. Nothing will prevail to part between them and their sins. Reprobate silver shall they be called, useless and worthless. When warnings, corrections, rebukes, and all means of grace, leave men unrenewed, they will be left, as rejected of God, to everlasting misery. Let us pray, then, that we may be refined by the Lord, as silver is refined.