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 on 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 whom it was given to hurt the earth and the 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. 5 Of the tribe of Juda were sealed twelve thousand. Of the tribe of Reuben were sealed twelve thousand. Of the tribe of Gad were sealed twelve thousand. 6 Of the tribe of Aser were sealed twelve thousand. Of the tribe of Nepthalim were sealed twelve thousand. Of the tribe of Manasses were sealed twelve thousand. 7 Of the tribe of Simeon were sealed twelve thousand. Of the tribe of Levi were sealed twelve thousand. Of the tribe of Issachar were sealed twelve thousand. 8 Of the tribe of Zabulon were sealed twelve thousand. Of the tribe of Joseph were sealed twelve thousand. Of the tribe of Benjamin were sealed twelve thousand. 9 After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands; 10 And cried with a loud voice, saying, Salvation to our God which sitteth upon the throne, and unto the Lamb. 11 And all the angels stood round about the throne, and about the elders and the four beasts, and fell before the throne on their faces, and worshipped God, 12 Saying, Amen: Blessing, and glory, and wisdom, and thanksgiving, and honour, and power, and might, be unto our God for ever and ever. Amen. 13 And one of the elders answered, saying unto me, What are these which are arrayed in white robes? and whence came they? 14 And I said unto him, Sir, thou knowest. And he said to me, These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb. 15 Therefore are they before the throne of God, and serve him day and night in his temple: and he that sitteth on the throne shall dwell among them. 16 They shall hunger no more, neither thirst any more; neither shall the sun light on them, nor any heat. 17 For the Lamb which is in the midst of the throne shall feed them, and shall lead them unto living fountains of waters: and God shall wipe away all tears from their eyes. Rev 7:1-17 (KJV)

 Appearing between the 5th and 6th seal will be the result of the preaching of the Two Witnesses - 144,000 believing Jews.

 It seems to me a no-brainer. Two guys who say they're Moses and Elijah, and able to duplicate the miracles of Moses and Elijah - you know, I'd say they're probably Moses and Elijah!

 Prior to the Rapture, a constant claim of heretics and cults was to claim they themselves are the 144,000. However, it's obvious from the passage these are all converted Jews. I'm picturing Jehovah's Witnesses and Seventh Day Adventists trying vainly to dispute with the actual 144,000. "You see, we're really the 144,000." "Um... no. We are. See this mark? Oh, you can't - you're not saved yet."

 the tribes listed are:

  1. Judah
  2. Reuben
  3. Gad
  4. Asher
  5. Naphtali
  6. Manassah
  7. Simeon
  8. Levi
  9. Zebulon
  10. Ephraim (Joseph)
  11. Benjamin
  12. Issachar

 The Bible lists the two half tribes of Ephraim and Manassah, which took the inheritance of Joseph. Ephraim is listed as Joseph, as Ephraim  was the inheritor of Joseph. This point suggests two possibilities:

  1. Genetic markers may be discovered by this time that identify the different Jewish tribes, finally settling the issue. As of the Babylonian Captivity of Jeremiah and the return under Nehemiah and Ezra, very few Jews could fully claim an inheritance in any particular tribe. Most simply referred to themselves as Judah (Jews), and only those with a lineage as Levi or Kohenim (priests) usually distinguished themselves as any other tribe. To claim you were of Naphtali or any other tribe required a Sefer Toldot, a scroll of genealogies.
  2. The more likely explanation is that either the Two Witnesses or the Holy Spirit Himself revealed the exact tribes each member was from upon conversion.

 The Tribe of Dan is not listed, which causes a great deal of speculation. It simply may be they are the most resistant to the truth. They are listed in the millennial kingdom, so that debunks some of the speculation.

 What is the visible seal?

 And the Lord said unto him, Go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry for all the abominations that be done in the midst thereof. Ezek 9:4 (KJV)

 The word "mark" is Tav, the last letter of the Hebrew alphabet. The Tav now is a block like three sided mark - but at the time this verse was written, it was shaped like a Cross. Many think that the Cross is the visible Seal. The "Left Behind" fiction series depicts it as invisible to the unsaved, which is a very real possibility. However, Satan is obviously aware of it, as he insists upon his own mark being placed upon his followers. So there is a very real probability it is visible to all.

 The 144,000 will create the largest revival on the face of the Earth. Millions will be saved, according to verse 9 (it has to be - if 144,000 are counted, then millions would be the count of the Great Multitude).

The most likely point of origin will be Jerusalem, where the Two Witnesses are preaching. The 144,000 may well spread out throughout the world from there, converting one person at a time.  If each of these people converted one person per day, they could reach as many as 181,440,000 people by the time they are martyred.

Here is another point to keep in mind, that your chances are not good. Many of the people converted by the 144,000 are martyred quickly. A great number appear to survive to the end of the tribulation, since there is a massive battle at the end of the tribulation.