Revelation 14 is the key Chapter that will help us resolve the Enigma about when the 144,000 were sealed from the 12 tribes of Israel. This Chapter starts with the vision of Christ standing on Mount Sion in Heaven with the 144,000 (please
note this is a vision of New Jerusalem). In this Chapter it can be determined that the 144,000 suffered Martyrdom, since they followed the Lamb wherever he goes and they were the Firstfruits. (Mt:16:24 - 25, Jn:12:24 - 26, Jn:13:36 - 37,
Mark 8: 34 - 35) (1Cor:15:21 - 26)

Read this related study:

144,000 sealed from the 12 tribes of Israel and Martyrdom
https://www.youtube.com/watch?v=5Zsf9ObT7sQ

Then in the same Chapter 14 it can be read the warnings of the 3 Angels:

"Fear God, and give glory to him; for the hour of his judgment is come".

"And there followed another angel, saying, Babylon is fallen, is fallen".

"And the third angel followed them, saying with a loud voice, If any man worship the beast and his image ... he shall be tormented with fire and brimstone".

Probably some readers of Revelation may have realized that these are the messages of the 3 angels sounding the 5th, 6th and 7th Trumpets, giving the
ominous warning that the Judgment of God is coming upon Jerusalem. And Jerusalem
is fallen, and whosoever had innocent blood on their hands and were involved in the Jerusalem Temple's worship of the Devil, and who also took part in the criminal activities of the "Synagogues of Satan", are going to Hell.

Then after the proclamation of the 3 angels, comes this very important key verse that will allow us to establish in time when the Martyrdom of the 144,000 followers of Christ took place:

Revelation 14:
13 And I heard a voice from heaven saying unto me, Write, Blessed are the dead which die in the Lord from henceforth: Yea, saith the Spirit, that they may rest from their labours; and their works do follow them.

From the sounding of the 6th Trumpet, parts of Jerusalem were already fallen, when the 7th Trumpet sounded, that was it: Jerusalem had fallen! So when we read above, "Blessed are the dead which die in the Lord from henceforth" (remember it was just after the proclamation of the 3 angels): it refers to the Martyrs of Christ who have died after the fall of Jerusalem.

Most Christians are unaware that after the fall of Jerusalem, the Early Church had returned to Jerusalem, and thrived through out the Holy Land. The thousand year reign with Christ refers to this Period between 70 AD and 1070 AD, when the Church
was still victorious over Satan, they loved not their lives unto death and through out that millennium, multitudes of Jewish Christians suffered Martyrdom in the land of Israel.

"be thou faithful unto death, and I will give thee a crown of life. He that hath an ear, let him hear what the Spirit saith unto the churches; He that overcometh shall not be hurt of the second death."

After 70 AD the Early Church remained "alive" for a 1000 year, it was only after that period the Church became "dead", overcome by Satan, poisoned with
Doctrines of Devils, it became pervaded with the stenching smell of Corruption and Decay. ("I know thy works, that thou hast a name that thou livest, and art dead.")

The Dimensions of New Jerusalem are 12,000 furlongs wide with the same measurement for Length and Height. The Thickness of the wall is 144 Cubits. The Number 144,000 comes from Revelation 22: (12 manner of fruits) x 12 months x 1000 years = 144,000:

Revelation 22:
1 And he shewed me a pure river of water of life, clear as crystal, proceeding out of the throne of God and of the Lamb.
2 In the midst of the street of it, and on either side of the river, was there the tree of life, which bare twelve manner of fruits, and yielded her fruit every month: and the leaves of the tree were for the healing of the nations.

Ezekiel 47:
12 And by the river upon the bank thereof, on this side and on that side, shall grow all trees for meat, whose leaf shall not fade, neither shall the fruit thereof be consumed: it shall bring forth new fruit
according to his months, because their waters they issued out of the sanctuary: and the fruit thereof shall be for meat, and the leaf thereof for medicine.

John 12:
24 Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit.
25 He that loveth his life shall lose it; and he that hateth his life in this world shall keep it unto life eternal.
26 If any man serve me, let him follow me; and where I am, there shall also my servant be: if any man serve me, him will my Father honour.

Make sure to read:

The Third Temple of Jerusalem. Prophet Ezekiel's Visions
https://www.youtube.com/watch?v=2w6LSFrxUaw

Very important Studies at this YouTube link:
https://www.youtube.com/user/vpyt7/videos