¹⁵ Again a message came to me from the Lord: ¹⁶ “Son of man, take a piece of wood and carve on it these words: ‘This represents Judah and its allied tribes.’ Then take another piece and carve these words on it: ‘This represents Ephraim and the northern tribes of Israel.’ ¹⁷ Now hold them together in your hand as if they were one piece of wood. ¹⁸ When your people ask you what your actions mean, ¹⁹ say to them, ‘This is what the Sovereign Lord says: I will take Ephraim and the northern tribes and join them to Judah. I will make them one piece of wood in my hand.’

²⁰ “Then hold out the pieces of wood you have inscribed, so the people can see them. ²¹ And give them this message from the Sovereign Lord: I will gather the people of Israel from among the nations. I will bring them home to their own land from the places where they have been scattered. ²² I will unify them into one nation on the mountains of Israel. One king will rule them all; no longer will they be divided into two nations or into two kingdoms. ²³ They will never again pollute themselves with their idols and vile images and rebellion, for I will save them from their sinful apostasy. I will cleanse them. Then they will truly be my people, and I will be their God.

²⁴ “David My servant shall be king over them, and they shall all have one shepherd; they shall also walk in My judgments and observe My statutes, and do them. ²⁵ Then they shall dwell in the land that I have given to Jacob My servant, where your fathers dwelt; and they shall dwell there, they, their children, and their children’s children, forever; and My servant David shall be their prince forever. ²⁶ Moreover I will make a covenant of peace with them, and it shall be an everlasting covenant with them; I will establish them and multiply them, and I will set My sanctuary in their midst forevermore. ²⁷ My tabernacle also shall be with them; indeed I will be their God, and they shall be My people. ²⁸ The nations also will know that I, the Lord, sanctify Israel, when My sanctuary is in their midst forevermore.” ’ ”

Ezekiel 37:15-28 (NKJV)

Israel was broken into two: Judah and the northern ten tribes of Israel. This happened when the ten northern tribes were sent into exile into Assyria. Those ten tribes never came back, but were scattered in among the Gentile nations. God promised to gather the two back together again under one King, in one land.

I say then, has God cast away His people? Certainly not! For I also am an Israelite, of the seed of Abraham, of the tribe of Benjamin. ² God has not cast away His people whom He foreknew. Or do you not know what the Scripture says of Elijah, how he pleads with God against Israel, saying, ³ “Lord, they have killed Your prophets and torn down Your altars, and I alone am left, and they seek my life”? ⁴ But what does the divine response say to him? “I have reserved for Myself seven thousand men who have not bowed the knee to Baal.” ⁵ Even so then, at this present time there is a remnant according to the election of grace.

Romans 11:1-5 (NKJV)

The breaking of the two allowed Gentiles to be grafted in (see Romans 11:16ff), with the 10 tribes being scattered as seed among the nations. Romans 11 is not only referring to Jews who had rejected Jesus. The rejection goes all the way back to the Assyrian exile and that which led up to it. But God would be faithful and call back His people…

Then I will sow her [the northern tribes] for Myself in the earth,
And I will have mercy on her who had not obtained mercy;
Then I will say to those who were not My people,
‘You are My people!’
And they shall say, ‘You are my God!’

Hosea 2:23 (NKJV)

And the grafted in Gentiles would be saved as well…

²² What if God, wanting to show His wrath and to make His power known, endured with much longsuffering the vessels of wrath prepared for destruction, ²³ and that He might make known the riches of His glory on the vessels of mercy, which He had prepared beforehand for glory, ²⁴ even us whom He called, not of the Jews only, but also of the Gentiles?

²⁵ As He says also in Hosea:

“I will call them My people, who were not My people,
And her beloved, who was not beloved.” [Hosea 2:23]
²⁶ “And it shall come to pass in the place where it was said to them,
‘You are not My people,’
There they shall be called sons of the living God.”

Romans 9:22-26 (NKJV)

Gentiles are grafted into the New Covenant from the first century onward, included in the blessings of Abraham…

¹³ Christ has redeemed us from the curse of the law, having become a curse for us (for it is written, “Cursed is everyone who hangs on a tree”), ¹⁴ that the blessing of Abraham might come upon the Gentiles in Christ Jesus, that we might receive the promise of the Spirit through faith.

Galatians 3:13-14 (NKJV), emphasis mine — compare to Ephesians 2:14-18 below

All this is laid out in Ephesians…

¹¹ Therefore remember that you, once Gentiles in the flesh—who are called Uncircumcision by what is called the Circumcision made in the flesh by hands— ¹² that at that time you were without Christ, being aliens from the commonwealth of Israel and strangers from the covenants of promise, having no hope and without God in the world. ¹³ But now in Christ Jesus you who once were far off have been brought near by the blood of Christ.

¹⁴ For He Himself is our peace, who has made both one, and has broken down the middle wall of separation, ¹⁵ having abolished in His flesh the enmity, that is, the law of commandments contained in ordinances, so as to create in Himself one new man from the two, thus making peace, ¹⁶ and that He might reconcile them both to God in one body through the cross, thereby putting to death the enmity. ¹⁷ And He came and preached peace to you who were afar off and to those who were near. ¹⁸ For through Him we both have access by one Spirit to the Father.

¹⁹ Now, therefore, you are no longer strangers and foreigners, but fellow citizens with the saints and members of the household of God, ²⁰ having been built on the foundation of the apostles and prophets, Jesus Christ Himself being the chief cornerstone, ²¹ in whom the whole building, being fitted together, grows into a holy temple in the Lord, ²² in whom you also are being built together for a dwelling place of God in the Spirit.

Ephesians 2:11-22 (NKJV), emphasis mine