Lets first look at Luke 4:
Luke 4:16-21 So He came to Nazareth, where He had been brought up. And as His custom was, He went into the synagogue on the Sabbath day, and stood up to read. And He was handed the book of the prophet Isaiah. And when He had opened the book, He found the place where it was written:
"The Spirit of the LORD is upon Me,
Because He has anointed Me
To preach the gospel to the poor;
He has sent Me to heal the brokenhearted
To proclaim liberty to the captives
And recovery of sight to the blind,
To set at liberty those who are oppressed;
To proclaim the acceptable year of the LORD."
Then He closed the book, and gave it back to the attendant and sat down. And the eyes of all who were in the synagogue were fixed on Him. And He began to say to them, "Today this Scripture is fulfilled in your hearing."
Jesus read this prophecy in Isaiah and was showing that He was fulfilling this prophecy. However, Jesus only read part of this prophecy, the portion He fulfilled at His first coming. Let's take a look at the whole passage in Isaiah:
Isaiah 61:1-6"The Spirit of the Lord GOD is upon me, Because the LORD has anointed me To preach good tidings to the poor; He has sent me to heal the brokenhearted, To proclaim liberty to the captives, And the opening of the prison to those who are bound; To proclaim the acceptable year of the LORD, And the day of vengeance of our God; To comfort all who mourn, To console those who mourn in Zion, To give them beauty for ashes, The oil of joy for mourning, The garment of praise for the spirit of heaviness; That they may be called trees of righteousness, The planting of the LORD, that He may be glorified." And they shall rebuild the old ruins, They shall raise up the former desolations, And they shall repair the ruined cities, The desolations of many generations. Strangers shall stand and feed your flocks, And the sons of the foreigner Shall be your plowmen and your vinedressers. But you shall be named the priests of the LORD, They shall call you the servants of our God.
First, we see that the rest of the prophecy will be fulfilled at "the day of vengeance of our God" aka the Tribulation. Most of the rest of this describes everything that will be restored leading up to the Millennium. Jesus will be doing all of those things, but look at the last sentence. There will also be a group of people that help Jesus do these things. They are called "the servants of our God" and will also "preach good tidings to the poor" aka the Gospel. This must be in the Tribulation, because the great commission of spreading the Gospel will be complete right before Jesus returns (the times of the Gentiles will be complete).
Now look at what the 144,000 are called:
Revelation 7:2-4 Then 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 whom it was granted to harm the earth and the sea, saying, "Do not harm the earth, the sea, or the trees till we have sealed the servants of our God on their foreheads." And I heard the number of those who were sealed. One hundred and forty-four thousand of all the tribes of the children of Israel.
The 144,000 are called "the servants of our God", the exact title given to those preparing for the Kingdom in the Tribulation as described by Isaiah.
Also, keeping in context, take a look at what John sees next after the introduction of the 144,000:
Revelation 7:9-12 After these things I looked, and behold, a great multitude which no one could number, of all nations, tribes, peoples, and tongues, standing before the throne and before the Lamb, clothed with white robes, with palm branches in their hands, and crying out with a loud voice, saying, "Salvation belongs to our God who sits on the throne, and to the Lamb!" All the angels stood around the throne and the elders and the four living creatures, and fell on their faces before the throne and worshiped God, saying: "Amen! Blessing and glory and wisdom, Thanksgiving and honor and power and might, Be to our God forever and ever. Amen."
Revelation 7:13- 17 Then one of the elders answered, saying to me, "Who are these arrayed in white robes, and where did they come from?" And I said to him, "Sir, you know.” So he said to me, "These are the ones who come out of the great tribulation, and washed their robes and made them white in the blood of the Lamb. Therefore they are before the throne of God, and serve Him day and night in His temple. And He who sits on the throne will dwell among them. They shall neither hunger anymore nor thirst anymore; the sun shall not strike them, nor any heat; for the Lamb who is in the midst of the throne will shepherd them and lead them to living fountains of waters. And God will wipe away every tear from their eyes."
I believe the saved multitude we see here in the same chapter as the 144,000 shows the fruit of their works of spreading the Gospel and ministering to people during the Tribulation.
Luke 21:28 "Now when these things begin to happen, look up and lift up your heads, because your redemption draws near.”