Jeremiah 51:20-23. God says to Jeremiah "You are my hammer and weapon of war", as He prepares to bring down Babylon by stirring up the Medes against them. God goes on to describe all of the ways He will use Jeremiah. "With you I destroy kingdoms; ... with you I break into pieces the chariot and the charioteer." Nations, men and women, the old and the young, the shepherd and his flock, the farmer and his team, governors and commanders, all will be broken due to Jeremiah.

Earlier in this chapter, God tells Jeremiah that he is planning on stirring up the Medes against Babylon and to tell the Jews to be ready to flee Babylon when it is under attack. How, then, is Jeremiah, a single prophet, the "hammer and weapon of war"? Isn't that really the Medes? What God is describing here, I think, is Jeremiah's true role - his power as a prophet in the times he was prophesying.

When people are called to and do speak God's word truthfully and bravely, no matter the consequences, they channel God's power. God's Word is are creative - it makes things happen. Jeremiah, being a true prophet, channeled this power when he spoke God's words at God's behest. So, just by speaking, by prophesying as God led him to do, Jeremiah actually became the hammer and the weapon of war, he brought down the power that destroyed chariots and nations, governors and commanders.

Even if God used the Mede army to accomplish this goal, it was Jeremiah speaking it truthfully that brought God's power into the world.