If one is to read Jeremiah 31:31-34 it is clear:
“31 “Behold, the days are coming, declares the Lord, when I will make a new covenant with the house of Israel and the house of Judah, 32 not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke,though I was their husband, declares the Lord. 33 For this is the covenant that I will make with the house of Israel after those days, declares the Lord: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people. 34 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.”
This scripture is applied to the future Israel. However the language of the writer of Hebrews in context, it is defined as the “believers” Jew or Gentile. The Gentile was grafted into the New Covenant based on the body of Jesus Christ forever and ever. Hebrews 10: 15-18:
15 And the Holy Spirit also bears witness to us; for after saying,
16 “This is the covenant that I will make with them
after those days, declares the Lord:
I will put my laws on their hearts,
and write them on their minds,”
17 then he adds,
“I will remember their sins and their lawless deeds no more.”
18 Where there is forgiveness of these, there is no longer any offering for sin.
Jesus said; Matthew 26:27-28:
27 And when He had taken a cup and given thanks, He gave it to them, saying, “Drink from it, all of you; 28 for this is My blood of the covenant, which is poured out for many for forgiveness of sins.