Who are the 144,000 mentioned in Revelation?

Here's what the book of Revelation says about the 144,000 in chapter 7:
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.

Notice that's it's not just 144,000 Jews; it's 12,000 from each tribe. The 144,000 are mentioned again in Revelation 14:
1 And I looked, and, lo, a Lamb stood on the mount Sion, and with him an hundred forty and four thousand, having his Father's name written in their foreheads.
2 And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder: and I heard the voice of harpers harping with their harps:
3 And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth.
4 These are they which were not defiled with women; for they are virgins. These are they which follow the Lamb whithersoever he goeth. These were redeemed from among men, being the firstfruits unto God and to the Lamb.
5 And in their mouth was found no guile: for they are without fault before the throne of God.
Here's what we know:
1. There are 12,000 from each tribe.
2. They're standing with Jesus on mount Sion.
3. They are sealed on their forehead.
4. They're singing a song that only they can learn.
5. They are male virgins.
6. They are without fault before the throne of God.

I want to point out point #2 and try to figure out where mount Sion is located. Here's what it says about mount Sion in Hebrews 12:22
But ye are come unto mount Sion, and unto the city of the living God, the heavenly Jerusalem, and to an innumerable company of angels,
So mount Sion is in the heavenly Jerusalem. You might not have heard that before, but the bible clearly says that there's a heavenly Jerusalem.

I believe these 144,000 are old testament saints that are currently in heaven. When the rapture happens, they'll be coming down the same time that we're going up (I believe that because they're mentioned right before the rapture in Revelation 7 & Revelation 14).  If you find that hard to believe, consider the fact that we're all coming down with Jesus when he comes to rule for 1000 years. They're just coming down a little earlier.
They will preach the gospel during the period of God's wrath along with the two witnesses which will be on the earth at that time as well.

I've heard pre-trib believers say that the 144,000 are Jews that will believe on Jesus when they see him come in the clouds at the rapture. Here's the problem with that. The tribes are gone. They've been totally mixed with the heathen. There's no way you could find 12,000 people that are of the tribe of Zabulon, much less 12,000 male virgin believers.
Many pre-trib believers think that there will be a mass repentance among the Jews during the tribulation.

Here's what Jesus said about that in John 5:43:

43 I am come in my Father's name, and ye receive me not: if another shall come in his own name, him ye will receive.

It sounds like they'll accept the anti-christ as their Messiah.  Consider the fact that he will have rebuilt the temple, brought peace to the middle east, been resurrected from the dead and he will be the ruler of the world. They'll probably think that he's fulfilling the prophecy of Isaiah 9:
6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.