John had several vision of the Fullness of Israel.
Then I heard the number of those who were sealed:
144,000 from all the tribes of Israel.
From the tribe of Judah 12,000 were sealed,
from the tribe of Reuben 12,000,
from the tribe of Gad 12,000,
from the tribe of Asher 12,000,
from the tribe of Naphtali 12,000,
from the tribe of Manasseh 12,000,
from the tribe of Simeon 12,000,
from the tribe of Levi 12,000,
from the tribe of Issachar 12,000,
from the tribe of Zebulun 12,000,
from the tribe of Joseph 12,000,
from the tribe of Benjamin 12,000 (Rev 7:4-8).
The actual numbers sealed are symbolic not literal. Ten symbolises completes and twelve represents Israel. 12 time 10 squared is a perfect symbol for the fullness of Israel. Twelve is also the number representing the apostles. This passage is a promise that Israel will fulfil the apostolic calling announced to Abraham (Gen 12:3) and Moses (Deut 4:6-7).
Then I looked, and there before me was the Lamb, standing on Mount Zion, and with him 144,000 who had his name and his Father’s name written on their foreheads. And I heard a sound from heaven like the roar of rushing waters and like a loud peal of thunder. The sound I heard was like that of harpists playing their harps. And they sang a new song before the throne and before the four living creatures and the elders. No one could learn the song except the 144,000 who had been redeemed from the earth. These are those who did not defile themselves with women, for they remained virgins. They follow the Lamb wherever he goes. They were purchased from among mankind and offered as firstfruits to God and the Lamb (Rev 14:1-4).
This is the fulfilment of the sealing seen in the earlier vision. It leads to a powerful advance of the gospel.
Then I saw another angel flying in midair, and he had the eternal gospel to proclaim to those who live on the earth—to every nation, tribe, language and people (Rev 14:6).
The eternal gospel will be proclaimed to all those who live on earth.