The gospel of the kingdom will be preached in all the world. And everyone will hear it, because it is a witness to all nations (:14). God selects a group of 144,000 Israelites from all 12 tribes of Israel for this. We find this described in the seventh seal.
Revelation 7
1 And after this I saw four messengers standing on the four corners of the earth (…)
2 And I saw another messenger, (…) and he cried with a loud voice to the four messengers, to whom it was given to hurt the land and the sea,
3 saying, Hurt not the land, neither the sea, nor the trees, until we have sealed the bondmen of our God in their foreheads.
4 And I heard the number of them which were sealed, an hundred and forty-four thousand sealed out of every tribe of the children of Israel.
the 144,000
These 144,000 Israelites are chosen from among believing Israel and sent into the world to proclaim the gospel of the kingdom. During this turbulent period, called the day of wrath (Rev. 6:17) or the day of vengeance (Isa. 61:1-2; Jer. 46:10), God’s judgments come upon the nations. In Revelation, these judgments are represented by seven trumpets (Rev. 8:6-9:21; Rev. 11:15-11:19). Before these judgments can cause harm, the 144,000 are sealed and protected from these judgments.
Matthew 24:14 only mentions that the gospel of the kingdom will be proclaimed throughout the inhabited world, but does not specify by whom. Later, in Matthew 24:31, this is discussed in more detail and we will return to the 144,000.
the sixth seal
The astute reader will notice that the sixth seal has been skipped in this explanation. In the sixth seal, an earthquake takes place and great signs in the heavens. Now, in Matthew 24:7, there is also mention of earthquakes (plural), but I am of the opinion that it does not concern the earthquake of Rev. 6:12, because that is mentioned very specifically: a great earthquake.
Revelation 6
12 And I saw when He opened the sixth seal, and there was a great earthquake; and the sun became black as sackcloth of hair, and the whole moon became as blood.
13 And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind.
Yet we also find the signs of the sixth seal mentioned in Matthew 24, but only later, in verse 29.
29 And immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken.
This is an important verse, because here we find another indication of time: immediately after the tribulation of those days. So we know that the sixth seal marks the end of the great tribulation. More about that later, of course.
What now follows from Matthew 24:15 is the second pericope, where the Lord in His speech goes into more detail about the period from the beginning of the great tribulation until the restoration of Israel (verses 15-30).