After the Church Age, Christians are raptured to heaven. Before the Tribulation, God calls and seals the 144,000 Hebrews from twelve tribes of Israel. God uses these witnesses to continue his purpose of redemption through Jesus Christ. Despite persecution, the 144,000 will proclaim the Gospel and millions of Tribulation Saints will be saved.

I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people, Saying with a loud voice, Fear God, and give glory to him; for the hour of his judgment is come: and worship him that made heaven, and earth, and the sea, and the fountains of waters (Revelation 14:6-7).

References: 21st Century Revelation: World Wars, Iraq Wars & End Wars - Chapter 21: Four Horses on the Run, Chapter 22: Keys to the Trumpets, Chapter 22: World Wars, Cold Wars, Chapter 23: Between Iraq and a Hard Place, Chapter 24: Iraq War III

21st Century Revelation's premise is that we are living in the End Times right before the Rapture. Today, we see some seal and trumpet judgments occur before the Tribulation. After the Rapture and before the Tribulation, four angels hold back the wind so the 144,000 can be sealed and protected to preach the mission of mercy. Despite the severe judgment of the Tribulation, God's primary purpose is the redemption of humanity.

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, Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads (Revelation 7:2-3).

Revelation 7 describes the 144,000 from the twelve tribes of Israel. At this point, Israel is united. All the tribes have 12,000 except Joseph has a double portion of 24,000 with Ephraim (Joseph) and Manasseh. Dan is not listed. All these witnesses are Hebrews not Gentiles.

I heard the number of them which were sealed: and there were sealed an hundred forty-four thousand of all the tribes of the children of Israel. Twelve thousand from each tribe (Revelation 7:4-5).

In Revelation 14, the 144,000 witnesses are described. They are sealed with their Father's name written in their foreheads, servants of God, virgins and undefiled, purposed to follow the Lamb, redeemed being firstfruits of the Tribulation Saints to God and spoke honestly, without fault.
 
These were redeemed from among men, being the firstfruits unto God and to the Lamb. And in their mouth was found no guile: for they are without fault before the throne of God (Revelation 14:4-5).

The 144,000 are evidence for a Rapture before the Tribulation. If there were hundreds of millions of Christians on earth, then they would be called to serve. Since Christians are caught up in the Rapture, God selects 144,000 Hebrews to be His preachers to Jews and all nations.

The dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord (1 Thessalonians 4:16-17).

Revelation is a time of justice and judgment but mercy and grace are available. God has given everyone plenty of opportunities to know and accept Him. The Holy Spirit convicts of sin and calls to salvation. But everyone comes to Jesus Christ by their own free will. Each person must respond, acknowledge our sin and accept God's perfect gift to be a Christian.

During the Tribulation, we see God's mercy and grace. People have the opportunity to repent and follow God. Some who accept Christ stand strong through severe persecution. Despite astonishing events and terrible judgments, many people still reject God but mercy and grace are available until the end.

I the Lord have called you in righteousness, and will hold your hand, and will keep you, and give you for a covenant of the people, for a light of the Gentiles (Isaiah 42:6).