The Newness of the New Covenant

In those days they shall no longer say: ‘The fathers have eaten sour grapes, and the children's teeth are set on edge.’ But everyone shall die for his own iniquity. Each man who eats sour grapes, his teeth shall be set on edge. Behold, the days are coming, declares the Lord, when I will make pa new covenant with the house of Israel and the house of Judah... And no longer shall each one teach his neighbor and each his brother, saying, ‘Know the Lord,’ for they shall all know me, from the least of them to the greatest, declares the Lord. For I will forgive their iniquity, and I will remember their sin no more.
Jeremiah 31:29-31, 34

What verse 34 is saying...in contrast to verses 29-30, is that in the old covenant, people became members of the covenant community simply by being born into that community.  As they grew up, some became believers in Yahweh and others did not.  This resulted in a situation within the covenant community where some members could urge others members to know the Lord.  In the new covenant community, however, one does not become a member by physical birth but rather by new birth, which requires faith on the part of every person.  Thus only believers are members of the new community: all members are believers, and only believers are members.  Therefore in the new covenant community there will no longer be a situation where some members urge other members to know the Lord.  There will be no such thing as an unregenerate member of the new covenant community.  All are believers, all know the Lord, because all have experienced the forgiveness of sins.  What Jeremiah is teaching in 31:33-34 is identical to what Isaiah is teaching in Isaiah 54:13: "all our children shall be taught by the LORD, and great shall be the peace of your children" (ESV).  Everyone in the covenant community will experience reconciliation (peace) with God, and so everyone will have a living relationship with the Lord, and so the divine instruction for living will be written upon the heart.

--Stephen J. Wellum and Peter J. Gentry, Kingdom through Covenant