The nation Israel did not accept the new covenant, the 3000 Jews who did at Pentecost is only a remnant that did, as Paul explained in Romans 11, which you quoted.
5 Even so then at this present time also there is a remnant according to the election of grace.
Their leaders killed Stephen, remember? Israel, the nation, is an enemy to God now (Romans 11:28), so that us gentiles can now be reconciled to God.
But we, the Body of Christ, are still not Israel, God still has a plan for Israel.
When Hebrews quoted Jeremiah, the author made it clear what that plan is going to be, in Hebrews 8:8
8 For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah:
"I will make" implies future tense. There is currently no new covenant between God and the nation of Israel now. It will be made after the Tribulation when the entire nation finally acknowledge Jesus as their promised Son of God.
You have failed to see that the author of the Book of Hebrews was quoting word-for-word the text from Jeremiah 31:31-34. The NKJV shows the OT text in Uppercase letters.
Verse 6 was written in the present tense during the first century.
Heb 8:6 But
now He has obtained a more excellent ministry, inasmuch as
He is also Mediator of a better covenant, which was established on better promises.
Heb 8:7 For if that first covenant had been faultless, then no place would have been sought for a second.
Heb 8:8 Because finding fault with them, He says: "BEHOLD, THE DAYS ARE COMING, SAYS THE LORD, WHEN I WILL MAKE A NEW COVENANT WITH THE HOUSE OF ISRAEL AND WITH THE HOUSE OF JUDAH—
Heb 8:9 NOT ACCORDING TO THE COVENANT THAT I MADE WITH THEIR FATHERS IN THE DAY WHEN I TOOK THEM BY THE HAND TO LEAD THEM OUT OF THE LAND OF EGYPT; BECAUSE THEY DID NOT CONTINUE IN MY COVENANT, AND I DISREGARDED THEM, SAYS THE LORD.
Heb 8:10 FOR THIS IS THE COVENANT THAT I WILL MAKE WITH THE HOUSE OF ISRAEL AFTER THOSE DAYS, SAYS THE LORD: I WILL PUT MY LAWS IN THEIR MIND AND WRITE THEM ON THEIR HEARTS; AND I WILL BE THEIR GOD, AND THEY SHALL BE MY PEOPLE.
Heb 8:11 NONE OF THEM SHALL TEACH HIS NEIGHBOR, AND NONE HIS BROTHER, SAYING, 'KNOW THE LORD,' FOR ALL SHALL KNOW ME, FROM THE LEAST OF THEM TO THE GREATEST OF THEM.
Heb 8:12 FOR I WILL BE MERCIFUL TO THEIR UNRIGHTEOUSNESS, AND THEIR SINS AND THEIR LAWLESS DEEDS I WILL REMEMBER NO MORE."
Heb 8:13 In that He says, "A NEW COVENANT," He has made the first obsolete. Now what is becoming obsolete and growing old is ready to vanish away.
You appear to be attempting to make the Two Peoples of God doctrine of modern Dispensational Theology work. It is a form of Dual Covenant Theology based on race. The doctrine's chief error is the claim that God did not fulfill His promises to the Jewish people at Calvary.
Jer_31:31 "Behold, the days are coming, says the LORD, when I will make a new covenant with the house of Israel and with the house of Judah—
Mat_26:28 For this is My blood of the new covenant, which is shed for many for the remission of sins.
Mar_14:24 And He said to them, "This is My blood of the new covenant, which is shed for many.
Luk_22:20 Likewise He also took the cup after supper, saying, "This cup is the new covenant in My blood, which is shed for you.
1Co_11:25 In the same manner He also took the cup after supper, saying, "This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me."
2Co_3:6
who also made us sufficient as ministers of the new covenant, not of the letter but of the Spirit; for the letter kills, but the Spirit gives life.
Heb_8:8 Because finding fault with them, He says: "BEHOLD, THE DAYS ARE COMING, SAYS THE LORD, WHEN I WILL MAKE A NEW COVENANT WITH THE HOUSE OF ISRAEL AND WITH THE HOUSE OF JUDAH—
Heb_8:13 In that He says, "A NEW COVENANT," He has made the first obsolete.
Now what is becoming obsolete and growing old is ready to vanish away.
Heb_9:15 And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance.
Heb_12:24 to Jesus the Mediator of the new covenant, and to the blood of sprinkling that speaks better things than that of Abel.
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