Who are the 144,000?

After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth to prevent any wind from blowing on the land or on the sea or on any tree. Then I saw another angel coming up from the east, having the seal of the living God. He called out in a loud voice to the four angels who had been given power to harm the land and the sea: "Do not harm the land or the sea or the trees until we put a seal on the foreheads of the servants of our God." 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.   Revelation 7:1-8

144,000 Jews are sealed from 12 tribes.  Apparently they are on earth when John receives the vision.

Judah is first, unlike other listing that have the oldest Reuben.  Since Jesus came from Judah, its ranking increased to the number one tribe.  This shows us that the work of Christ is important to understanding who they are.  This is a New Testament view of these people, not an Old Testament genealogy.

Joseph is mentioned twice—his own tribe (probably Ephraim not mentioned by name since it fell in idolatry) and the half-tribe of Manasseh.  Dan is missing, probably because during the split of Israel, it sided with the North and became engaged in pagan practices. 
These 144,000 show up later in Revelation:

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 kept themselves pure. They follow the Lamb wherever he goes. They were purchased from among men and offered as firstfruits to God and the Lamb. No lie was found in their mouths; they are blameless.  Revelation 14:1-5

So who are the 144,000?  What we know...
·      They are sealed which means their future relationship is secured.  They are “locked in.”  Nobody but the king can unseal it.
·      They are marked by Jesus’ name and the Father’s name.  This is not a literal mark, but since they profess the name of Jesus and call themselves followers, Jesus makes them part of the family through an unbroken bond.
·      They sing their own special song, a praise song.
·      They have not defiled themselves with women.  They are pure.  They have never lied.  They can’t be blamed for anything. 
·      They are followers of the Lamb (Jesus).

 QUESTIONS to consider:
·      Are they all Jews?  Their lineage is given, so no reason to doubt that.
·      Did they really never sin?  Are they 144,000 Jesuses?  They must have sinned.  We have all sinned.  Maybe it's saying they haven’t defiled themselves with other faiths.  Maybe they are forgiven and cleansed of their sin.
·      Could this be a literal number?  Is it just a coincidence that exactly 12,000 from each tribe are saved.  Sounds like a quota.  What happens to the 12,001 person?  Does God cut them off.  The number must mean something else.

Who could they be?
They are Jews who are saved and sealed because they believe in Christ.  Two possible formulas for the number.

12 tribes (OT lineage) x 12 (itself for emphasis) x 1,000 (lots) = 144,000.

This formula is a formula of emphasis.  "There are lots of Jews who were saved from each tribe."

12 tribes (OT heritage) x 12 apostles (NT belief) x 1,000 (lots) = 144,000.

The 144,000 could be ALL JEWISH BELIEVERS (Messianic) who are saved and later will be in heaven.