Jeremiah 1:18
For, behold, I have made thee this day a defenced city, and an iron pillar, and brasen walls against the whole land, against the kings of Judah, against the princes thereof, against the priests thereof, and against the people of the land.
God made Jeremiah steadfast and protected on that day, like a fortified city and an iron pillar and walls of bronze. That defense stood against the whole land, against the kings of Judah and its officials and rulers, against its priests, and against the people of the land. The promise enabled and assured him to speak God's message faithfully in the face of their opposition and resistance. They could not overcome him because God was with him.