John Gill

For, behold, I have made thee this day a defenced city,.... Or, "as" one; so read the Targum, Septuagint, Syriac, and Arabic versions; which is inexpungible, and cannot be taken: and an iron pillar; which cannot be removed out of its place: and brasen walls; which cannot be broken down. All these metaphors show the safety and security of the prophet, being surrounded by the power of God; his constancy, immovableness, and invincibleness in the work of the Lord, having such a spirit of power, fortitude, and of a sound mind, that nothing was able to move and shake him, or to deter him from the execution of his office; and that he should stand inflexible against the whole land; of Judea, and all the inhabitants of it: against the kings of Judah; in successive reigns, as Jehoahaz, Jehoiakim, Jehoiachin, or Jechonias, and Zedekiah: against the princes thereof; who desired he might be put to death, Jer 38:4, against the priests thereof; who all of them dealt falsely, and were given to covetousness, Jer 8:10, and against the people of the land; who were grievously addicted to idolatry, and all manner of wickedness.