Revelation 7 (KJV)
1.And after these things I saw four angels standing on the four corners of the earth, holding the four winds of the earth, that the wind should not blow on the earth, nor the sea, nor on any tree. 2. And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whim it was given to hurt the earth and sea.
3. Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads. 4. And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel.
Let me point out that even in these times, most knew the earth was a sphere and not flat. I learned this all through public school saying that Christopher Columbus went a sailing the ocean blue in 1492 to prove the earth was "round" and not flat. This is such a heaping helping of malarkey that has propagated ignorance since then. It has given athiests reason to try and disprove the bible when common sense should dictate this is a figure of speech, Ahem.
Rather than go through the who chapter this time, I will point out that there are 12 tribes of Israel, but two are missing in this list. Dan and Ephriam are the ones being left out of this sealing. They're being replaced by the tribe of Joseph and I believe Judah. This by no means says that individuals of these tribes won't receive salvation, because Dan and Ephraim WILL receive land in the Millenial Kingdom.
For anyone who says "but we don't know who's in what tribe" I would reply, "but God does." Depite the many persecutions and attempts to wipe the Jews out forever, God has kept them as a distinct race of people. They still have work to do and 144,000 Jews will carry on with His work during the tribulation period. They will suffer for sure, but until God is done, these tireless soldiers for Christ will fight on.