What does it mean that “the fathers have eaten sour grapes” (Jeremiah 31:29)?

In Jeremiah 31 the prophet refers to a proverb that people used in his day. It went like this:
The proverb (which also appears in Ezekiel 18:2) has to do with responsibility. The people who were experiencing hardship pointed to others as the cause; they said, “God is judging us because of the sins of our fathers.”

The context is God’s announcement that He will make a new covenant with Israel and Judah (Jeremiah 31:31), in contrast to the Old Covenant (Jeremiah 31:32). One of the characteristics of the Old Covenant (or the Mosaic Covenant) was its conditional nature. If Israel obeyed God, He would allow the nation to live in the land in peace. If, on the other hand, Israel disobeyed God and broke the covenant, God would judge them and remove the nation from the land. The blessings and cursings were corporate: the whole nation rose or fell together. So, to many Israelites, it seemed like the proverb was true: God would punish “the children for the sin of the parents to the third and fourth generation” (Exodus 20:5). In their use of the proverb, however, they were ignoring personal responsibility.

Jeremiah had been sent to proclaim the inevitability of God’s judgment and the people’s exile from the land. The people blamed King Manasseh’s sin—he was the “father” who had eaten the sour grapes—and they excused themselves—it was their teeth set on edge. But God also announces, through Jeremiah, that days were coming when He would bring His people back to the land of Israel, and He would watch over them “to build and to plant” rather than to destroy (Jeremiah 31:28, ESV). The restoration would include individual blessings for individual people who were of Israel (Jeremiah 31:25). God would bring people and beasts back into the land (Jeremiah 31:27).

God explains that in those days of restoration it will no longer be said that the fathers have eaten sour grapes and the children’s teeth are set on edge (Jeremiah 31:29). The judgment would be removed, and the people would see that each individual is held accountable for his own iniquity. Blame-shifting would cease. The one who eats the sour grapes will have to deal with the consequences himself (Jeremiah 31:30).

In the Mosaic Covenant, the actions of the fathers did, to an extent, impact the sons’ ability to enjoy the land. What one generation did could indeed impact the next. The fathers would eat sour grapes, and the children’s teeth were set on edge. Because of previous generations of failure, the nation of Israel endured exile in Babylon. But it was not right for the children of Israel to cast off all responsibility. They spouted the proverb as if they bore no guilt at all. But it was their sin that also contributed to the judgment.

God explains that the New Covenant with Israel and Judah will bring a different perspective (Jeremiah 31:31–32). Each person will be accountable for himself, and God will provide him with righteousness. God promises,
In addition, each person under the New Covenant will know God, and all will be forgiven of their sin (Jeremiah 31:34). Under the Mosaic Covenant, Israel was accountable as a nation for their sin. Under the New Covenant, God will hold each individual accountable and provide for each person’s righteousness. No longer would Israel have occasion to say that the fathers have eaten sour grapes and the children’s teeth are set on edge.

The New Covenant that God promises to make for Israel and Judah is unconditional, being based purely on the work of God. He will accomplish individual responsibility and individual righteousness by forgiving the people’s sins. Jesus ratified this covenant when He died on the cross. With His shed blood, He paid for the sins of all who would receive the blessings of the New Covenant (Israel and Judah)—and He paid for the sins of all the rest of humanity, in fulfillment of God’s promise to Abraham (Genesis 12:3b).

God is faithful and trustworthy. Even though the fathers may have eaten sour grapes, no longer will the children’s teeth be set on edge. Each person is accountable before God, and each person can be righteous in God’s sight by putting his or her belief in Jesus.