Also in the OT is this determination below, being left 'few in numbers' is having a population of 144,000 on the day of return, joining them via resurrection of the dead is the crowd mentioned in Eze:37 so that is who the whole House of Israel will be for the 1,000 year period. Also there is 1/3 of the Gentiles alive at that time and 2/3 die that same day. Joining them via resurrection from the grave are Christian Gentiles going back to Acts:10.

Eze:39:22:
So the house of Israel shall know that I am the LORD their God from that day and forward.
Eze:39:23:
And the heathen shall know that the house of Israel went into captivity for their iniquity:
because they trespassed against me,
therefore hid I my face from them,
and gave them into the hand of their enemies:
so fell they all by the sword.
Eze:39:24:
According to their uncleanness and according to their transgressions have I done unto them,
and hid my face from them.
Eze:39:25:
Therefore thus saith the Lord GOD;
Now will I bring again the captivity of Jacob,
and have mercy upon the whole house of Israel,
and will be jealous for my holy name;
Eze:39:26:
After that they have borne their shame,
and all their trespasses whereby they have trespassed against me,
when they dwelt safely in their land,
and none made them afraid.


Joining the 12 Tribes would be Pharaoh and all of Egypt as well as all the races where children were taken captive during the exodus as being the group that 'was beheaded for the word of God' in Re:20:4. Along with them would be some other Gentiles in the OT that God has plans for because Jesus is a Priest that covers everybody including Adam.

Zec:13:7:
Awake,
O sword,
against my shepherd,
and against the man that is my fellow,
saith the LORD of hosts:
smite the shepherd,
and the sheep shall be scattered:
and I will turn mine hand upon the little ones.
Zec:13:8:
And it shall come to pass,
that in all the land,
saith the LORD,
two parts therein shall be cut off and die;
but the third shall be left therein.
Zec:13:9:
And I will bring the third part through the fire,
and will refine them as silver is refined,
and will try them as gold is tried:
they shall call on my name,
and I will hear them:
I will say,
It is my people:
and they shall say,
The LORD is my God.

(LORD in all caps is the one true God from Ge:1)

Zec:14:16:
And it shall come to pass,
that every one that is left of all the nations which came against Jerusalem shall even go up from year to year to worship the King,
the LORD of hosts,
and to keep the feast of tabernacles.