"Therefore this is what the Lord God Almighty says: 'Because the people have spoken these words, I will make my words in your mouth a fire and these people the wood it consumes.'" NIV translation
Fire refines and here the words the people spoke will be refined and the truth made known. They had said that God would not judge them for their idolatry but God's action would demonstrate the truth.
"'People of Israel,' declares the Lord, 'I am bringing a distant nation against you - an ancient and enduring nation, a people whose language you do not know, whose speech you do not understand. Their quivers are like an open grave; all of them are mighty warriors. They will devour your harvests and food, devour your sons and daughters; they will devour your flocks and herds, devour your vines and fig trees. With the sword they will destroy the fortified cities in which you trust.'" NIV translation
Once again, God lets Judah know that it will be Babylon that is used to judge them. The judgment would affect everything from their children to their fields and cities.
"'Yet even in those days,' declares the Lord, 'I will not destroy you completely. And when the people ask, "Why has the Lord our God done all this to us?" you will tell them, "As you have forsaken me and served foreign gods in your own land, so now you will serve foreigners in a land not your own."'" NIV translation
Once again, we see that God promises to spare a remnant from the coming destruction. We also see that this remnant is left for a reason which is to tell others about what God has done. There would be no doubt as to who brought on the judgment and that it was deserved (God was just in His judgment).
"'Announce this to the descendants of Jacob and proclaim it in Judah: Hear this, you foolish and senseless people, who have eyes but do not see, who have ears but do not hear: Should you not fear me?' declares the Lord. 'Should you not tremble in my presence? I made the sand a boundary for the sea, an everlasting barrier it cannot cross. The waves may roll, but they cannot prevail; they may roar, but they cannot cross it.'" NIV translation
God uses the example of the sea to remind the people that he is the one who created all things and even the earth obeys Him. As the Creator, it is only right that God should tell the creation how it is to operate.