Jeremiah 27:18
But if they be prophets, and if the word of the LORD be with them, let them now make intercession to the LORD of hosts, that the vessels which are left in the house of the LORD, and in the house of the king of Judah, and at Jerusalem, go not to Babylon.
The challenge was aimed at the prophets who were predicting that the vessels of the temple already taken to Babylon would soon be returned. If they truly were prophets and the word of the Lord was truly with them, they were to make intercession to the Lord of hosts. That intercession was to be for the vessels still left in the house of the Lord, and in the house of the king of Judah, and at Jerusalem, that those vessels would not go to Babylon.
The demand tested their claim to be prophets against the judgment already announced. The intercession called for was a plea that the threatened vessels be spared and allowed to stay in place rather than also being taken. They would remain in place only if God granted that plea. The vessels already taken, by contrast, were to remain in Babylon until the seventy years ended.