Many Reformed believers are familiar with the potter and clay analogy in Romans 9, which speaks to God's absolute sovereignty. However, many Reformed believers are less familiar with the potter and clay analogy in Jeremiah 18. The two analogies are only loosely related. Jeremiah 18 also points out God's sovereignty, but with a different signification. In Romans 9, you will recall, the signification is that God can make a man into any kind of vessel he wishes. In Jeremiah 18 the point is that any nation's future is malleable in the hands of God: just because things are one way now doesn't mean they won't change.
Here is the text.
Then the word of the LORD came to me, saying,
O house of Israel, cannot I do with you as this potter? saith the LORD. Behold, as the clay is in the potter's hand, so are ye in mine hand, O house of Israel.
At what instant I shall speak concerning a nation, and concerning a kingdom, to pluck up, and to pull down, and to destroy it; if that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them.
And at what instant I shall speak concerning a nation, and concerning a kingdom, to build and to plant it; if it do evil in my sight, that it obey not my voice, then I will repent of the good, wherewith I said I would benefit them.
Now therefore go to, speak to the men of Judah, and to the inhabitants of Jerusalem, saying, Thus saith the LORD; Behold, I frame evil against you, and devise a device against you: return ye now every one from his evil way, and make your ways and your doings good.
And they said, There is no hope: but we will walk after our own devices, and we will every one do the imagination of his evil heart.
Therefore thus saith the LORD; Ask ye now among the heathen, who hath heard such things: the virgin of Israel hath done a very horrible thing. Will a man leave the snow of Lebanon which cometh from the rock of the field? or shall the cold flowing waters that come from another place be forsaken? Because my people hath forgotten me, they have burned incense to vanity, and they have caused them to stumble in their ways from the ancient paths, to walk in paths, in a way not cast up; to make their land desolate, and a perpetual hissing; every one that passeth thereby shall be astonished, and wag his head. I will scatter them as with an east wind before the enemy; I will shew them the back, and not the face, in the day of their calamity.
Notice that point of the passage is two-fold. On the one hand, if a nation is doing uprightly at one point, and God is blessing them, that does not mean that God will continue to bless them. In fact, if they fall into rebellion against God, God will chastise them. In other words, God is a holy God.
The other point is converse. If a nation is in rebellion against God and on the path to destruction, if they repent, they will be spared. In other words, God is also a God of mercy.
Both sides of this coin were graphically illustrated in the prophet Jonah and his ministry to Ninevah. He was a prophet of God but he rebelled and was chastised. Ninevah was in rebellion against God but repented and was spared.
This passage about nations is also applicable to individuals. We see the same theme developed more particularly for individuals in Ezekiel 18 (which I've discussed here). That's why I have felt free to illustrate God's chastisement on Jonah. I could instead have simply pointed to the time of the judges. In those days, the people of Israel repeated fell into wickedness, and God sent them chastisement in the form of oppression by neighboring kings and nations. Then, when they repented, God sent them judges to relieve their chastisement.
Yet, although this passage can be applied to individuals, its primary significance is about nations. It helps to explain why Ninevah was spared and why (as explained in more detail in Romans 11) the nation of Israel is presently under God's chastisement. The land of Ninevah repented, but the nation of Israel rebelled.
Some Christians, even those who have a Reformed soteriology, believe that part of God's character of dealing with nations as nations has ceased. I'd respectfully disagree. There does not seem to be any hint in Scripture that is not longer true that "if that nation, against whom I have pronounced, turn from their evil, I will repent of the evil that I thought to do unto them."
There are many nations presently in rebellion against God and subject to his chastisement. The United States and the United Kingdom come to mind. Now is the time for national repentance in those lands, if those nations wish to avert the judgment of God for their rebellion against Him.
And even more so, within those lands and throughout the world there are many individuals who likewise are rebelling against God. Those people ought to recognize that they are presently under the wrath and judgment of God for their sin. If they continue on their present course, God will consign them to the eternal torments of hell. But it is not hopeless. If they will repent of their sins and trust in Christ alone for salvation, they will be saved.