That will occur in the millennium where the new dispensation will begin which is Christ ruling and reigning upon earth. Your quote: "(the prophetic name of the ten tribes used by Isaiah and others) would become the fullness of the Gentiles". You're going to have to show me a reference on that one, as I'm not finding it.
Follow the scriptures I'm going to quote now - and concentrate:
1. THE PROMISE
"And Abram fell on his face: and God talked with him, saying, As for me, behold, my covenant is with thee, and thou shalt be a father of
hamon goyim (many nations). Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of
hamon goyim have I made thee." (Genesis 17:4-5)
2. THE SEED OF EPHRAIM
"And when Joseph saw that his father (
Jacob - the father of Israel) laid
his right hand upon the head of Ephraim, it displeased him: and he held up his father’s hand, to remove it from Ephraim’s head unto Manasseh’s head. And Joseph said unto his father, Not so, my father: for this is the firstborn; put thy right hand upon his head.
And his father refused, and said, I know it, my son, I know it: he (Manasseh) also shall become a people, and he also shall be great: but truly his younger brother (Ephraim) shall be greater than he, and his seed shall become
mlo hag goyim (the fullness of the Gentiles) - a multitude of nations. And he blessed them that day, saying, In thee shall Israel bless, saying, God make thee as Ephraim and as Manasseh:
and he set Ephraim before Manasseh." (Genesis 48:17-20)
The ten northern tribes are prophetically called Ephraim in the prophets
- and Ephraim ceased being a nation before God (one nation before God)
when they conspired with Assyria against Judah (against the Jews and Benjamites):
"And it was told the house of David, saying, Syria is confederate with Ephraim. And his heart was moved, and the heart of his people, as the trees of the wood are moved with the wind.
Then said the LORD unto Isaiah, Go forth now
to meet Ahaz, thou, and Shear–jashub thy son, at the end of the conduit of the upper pool in the highway of the fuller’s field;
And say unto him, Take heed, and be quiet;
fear not, neither be fainthearted for the two tails of these smoking firebrands, for the fierce anger of Rezin with Syria, and of the son of Remaliah
(because Syria, Ephraim, and the son of Remaliah, have taken evil counsel against thee, saying,
Let us go up against Judah, and vex it, and let us make a breach therein for us, and set a king in the midst of it, even the son of Tabeal):
Thus saith the Lord GOD,
It shall not stand, neither shall it come to pass. For
the head of Syria is Damascus, and the head of Damascus is Rezin;
and within threescore and five years shall Ephraim be broken, that it be not a people." (Isaiah 7:2-8).
The ten tribes instead found themselves being attacked by Assyria and were exiled and never returned -
and their seed have intermarried with Gentiles in the nations, ever since. Hosea wrote about them:
"Then said God, Call his name Lo–ammi:
for ye are not my people, and I will not be your God." (Hosea 1:9 - God
was not talking to Judah - He was talking to the ten northern tribes, a.k.a Ephraim).
Then Hosea wrote:
"Yet the number of the children of Israel shall be as the sand of the sea, which cannot be measured nor numbered; and it shall come to pass, that in the place where it was said unto them, Ye are not my people, there it shall be said unto them, Ye are the sons of the living God." (Hosea 1:10).
Before we quote the rest of the prophecy (which appears also in Ezekiel and in other prophets), take note that Paul quoted the above prophecy, saying,
"Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth. Thou wilt say then unto me, Why doth he yet find fault? For who hath resisted his will? Nay but, O man, who art thou that repliest against God? Shall the thing formed say to him that formed it, Why hast thou made me thus? Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour?
What if God, willing to shew his wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction: And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory,
- Even us, whom he hath called, not of the Jews only, but also of the Gentiles? As he saith also in Osee,
I will call them my people, which were not my people; and her beloved, which was not beloved. And it shall come to pass, that in the place where it was said unto them, Ye are not my people; there shall they be called the children of the living God.
Esaias also crieth concerning Israel, Though the number of the children of Israel be as the sand of the sea,
a remnant shall be saved" (Romans 9:18-27).
Hosea closed with another prophecy, saying:
"Then shall the children of Judah and the children of Israel be gathered together, and appoint themselves one head" (Hosea 1:11)
The rest of verse 11 is a repeat of what Hosea began to talk about in Hosea 1:4, saying,
"And the LORD said unto him, Call his name Jezreel;
for yet a little while, and I will avenge the blood of Jezreel upon the house of Jehu, and
will cause to cease the kingdom of the house of Israel. And it shall come to pass at that day, that I will break the bow
of Israel in the valley of Jezreel." (Hosea 1:4-5).
God was not talking to Judah (the southern kingdom). The 10 northern tribes were defeated by the Assyrian king
in the valley of Jezreel, exiled and scattered among the nations. And the kingdom
of Israel (Ephraim - the northern kingdom) - ceased to exist.
The southern kingdom (Judah, i.e the Jews and the tribe of Benjamin - continued to exist).
The kingdom of Israel ceased to exist over 150 years before the later exile of Judah (the Jews) to Babylon.
So Hosea closes verse 11 with what he began to say about Israel coming up out of the land (being exiled):
"and they shall come up out of the land: for great shall be the day of Jezreel."
But Hosea said even more - something which was a mystery until Christ came:
"Then shall the children of Judah and the children of Israel be gathered together, and appoint themselves one head" (see Hosea 1:1-11)
Jeremiah wrote:
"They shall come with weeping, and with supplications will I lead them: I will cause them to walk by the rivers of waters in a straight way, wherein they shall not stumble:
for I am a father to Israel, and Ephraim is my firstborn. (Jeremiah 31:9)
Ezekiel wrote:
"Moreover, thou son of man, take thee one stick, and write upon it,
For Judah, and for the children of Israel his companions: then take another stick, and write upon it,
For Joseph, the stick of Ephraim, and for all the house of Israel his companions: And join them one to another into one stick;
and they shall become one in thine hand.
And when the children of thy people shall speak unto thee, saying, Wilt thou not shew us what thou meanest by these? Say unto them,
Thus saith the Lord GOD; Behold, I will take the stick of Joseph, which is in the hand of Ephraim, and the tribes of Israel his fellows, and will put them with him, even
with the stick of Judah, and
make them one stick, and they shall be one in mine hand. And the sticks whereon thou writest shall be in thine hand before their eyes.
"And say unto them, Thus saith the Lord GOD; Behold,
I will take the children of Israel from among the heathen, whither they be gone, and will gather them on every side, and bring them into their own land: And I will make them one nation in the land upon the mountains of Israel; and one king shall be king to them all: and they shall be no more two nations, neither shall they be divided into two kingdoms any more at all" (Ezekiel 37:16-22).
"There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female:
for ye are all one in Christ Jesus. And
if ye be Christ’s, then are ye Abraham’s seed, and heirs according to the promise." (Galatians 3:28-29).
"And Abram fell on his face: and God talked with him, saying, As for me, behold, my covenant is with thee, and thou shalt be a father of
hamon goyim (many nations). Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of
hamon goyim have I made thee." (Genesis 17:4-5). "And I will give unto thee,
and to thy seed after thee, the land wherein thou art a stranger, all the land of Canaan, for an everlasting possession;
and I will be their God." (Genesis 17:8).