Why are the mountains of brass significant in Zechariah?

In Zechariah 6, the mountains of brass symbolize the immovable and unchanging decrees of God. They serve as a powerful illustration of God's authority as He sends out His messengers and accomplishes His will across the earth. These mountains remind us of God's sovereign control over history and creation. The four chariots that emerge from these mountains represent God's active involvement in the world, carrying forth His judgment and grace. Thus, the mountains of brass emphasize the reality that God's purposes stand firm and are not influenced by the uncertainties of worldly events.