Some people who are against preaching the gospel to the world say that Matthew 24:14 is not a command to preach the gospel, but nothing more than a prophecy which says that the gospel will be preached, and therefore should not be viewed as a command, commission, or instruction for the Church to preach the gospel.
Matthew 24:14 says "And this gospel of the kingdom will be preached in all the world as a witness to all the nations, and then the end will come".
On the surface, this seems to just be a prophecy. But when you look at God's instructions and examples further, it becomes apparent that this is also an instruction for the Church to preach the gospel to the world as a witness as best the Church of God is able.
We are commanded to love God with all our being (Matthew 22:37-38). Jesus set an example for us to follow. He sought to do God's will and finish His work in every way. "Jesus said to them, 'My food is to do the will of Him who sent Me, and to finish His work' " (John 4:34).
Matthew 24:14 shows that it is God's will that the gospel be preached in all the world as a witness before the end. We know we are near the end. We have the opportunity, the freedom, to preach the gospel to the world as a witness. As we have ability, we should do it to fulfill God's will.
Matthew 24:14 therefore reinforces other commands in the New Testament to preach the gospel.
Did Jesus ever do something in order to help fulfill prophecy? If so, He set an example for us to follow. If Christ looked to prophecy to know the Father's will, and then make choices to help fulfill prophecy, then we should do likewise. We should do what we are able to do to fulfill Matthew 24:14 knowing that Matthew 24:14 shows that it is the Father's will that the gospel be preached in all the world as a witness before the end.
"Now His betrayer had given them a sign, saying, 'Whomever I kiss, He is the One; seize Him.' Immediately he went up to Jesus and said, 'Greetings, Rabbi!' and kissed Him. But Jesus said to him, 'Friend, why have you come?' Then they came and laid hands on Jesus and took Him. And suddenly, one of those who were with Jesus stretched out his hand and drew his sword, struck the servant of the high priest, and cut off his ear. But Jesus said to him, 'Put your sword in its place, for all who take the sword will perish by the sword. Or do you think that I cannot now pray to My Father, and He will provide Me with more than twelve legions of angels? How then could the Scriptures be fulfilled, that it must happen thus?' In that hour Jesus said to the multitudes, 'Have you come out, as against a robber, with swords and clubs to take Me? I sat daily with you, teaching in the temple, and you did not seize Me. But all this was done that the Scriptures of the prophets might be fulfilled.' " (Matthew 26:48-56).
Notice that Christ restrained the one who tried to protect Him by striking the servant of the high priest. He stated that He could call on His Father's angels to protect Him. But He did not do that for one specific reason. He said, "How then could the Scriptures be fulfilled?" Jesus did not look at prophecy in a passive sense. He actively made decisions and choices to help fulfill prophecy. He knew that it was His Father's will that prophecy be fulfilled, and He made it His responsibility to take an active part in making sure those prophecies were fulfilled.
Earlier, at supper, Jesus spoke of Judas who was to betray Him: "If you know these things, blessed are you if you do them. I do not speak concerning all of you. I know whom I have chosen; but that the Scripture may be fulfilled, 'He who eats bread with Me has lifted up his heel against Me.' Now I tell you before it comes, that when it does come to pass, you may believe that I am He" (John 13:17-19).
Jesus knew Judas would betray Him, but He chose him as a disciple anyway. Why? To fulfill prophecy, "that the Scripture may be fulfilled". Jesus deliberately chose one He knew would betray Him so that prophecy would be fulfilled.
There are many scriptures that show that Jesus did this or that thing so that prophecy would be fulfilled. Look at Matthew 4:13-16 for example: "And leaving Nazareth, He came and dwelt in Capernaum, which is by the sea, in the regions of Zebulun and Naphtali, that it might be fulfilled which was spoken by Isaiah the prophet, saying: 'The land of Zebulun and the land of Naphtali, By the way of the sea, beyond the Jordan, Galilee of the Gentiles: The people who sat in darkness have seen a great light, And upon those who sat in the region and shadow of death Light has dawned' ".
At first reading, it might seem that Jesus may have chosen to dwell in regions of Zebulun and Naphtali but not necessarily for the purpose or even with the awareness that He was fulfilling prophecy. But that is unlikely. He knew the scriptures concerning Himself. He had been studying and asking questions from age twelve (Luke 2:42-49). It is apparent, as Matthew 26:48-56 and John 13:17-19 show, that it was Jesus's practice to choose His actions for the purpose of doing His Father's will regarding the fulfillment of prophecy. He didn't just "let things happen". He did His part to make sure prophecy was fulfilled. For other examples, see Matthew 8:16-17, Matthew 12:15-21, Matthew 13:34-35, Matthew 21:1-6, Luke 4:16-21, Luke 18:31-33, and John 18:7-9.
Jesus was not a passive observer watching prophecy be fulfilled in Himself. He actively did His part to fulfill prophecy. He used Old Testament prophecy to know the Father's will, and He let scriptural prophecy guide His choices and decisions. Fulfilling prophecy was for Jesus Christ part of doing the Father's work.
As Christ chose Judas as a disciple whom He knew would betray Him in order to fulfill prophecy concerning Judas, so we should preach the gospel to help fulfill a prophecy we know about, the prophecy that the gospel will be preached in all the world as a witness (Matthew 24:14). As Jesus restrained those who would fight to protect Him from being arrested so that prophecy would be fulfilled, so we must do what we can to help fulfill the prophecy in Matthew 24:14. As Jesus used prophecy to know the Father's will, and then do it, setting us an example, so we should use Matthew 24:14 as a guide to know the Father's will, then do it.
By itself, Matthew 24:14 is not a command. But putting this prophecy together with other passages in the Bible that show we are to do God's will in everything and the example of Christ of working to fulfill prophecy, it becomes the equivalent of a command to preach the gospel to the world. As we love God, we will seek to do His will, and as we understand that it is God's will that the gospel be preached to the world, we will do it.
That makes Matthew 24:14, not just a prophecy that we are to passively observe, but the equivalent of a command to actively support the fulfillment of this prophecy by preaching the gospel to the world as a witness as best we are able.
Matthew 24:14 therefore supports and reinforces the commission to the Church to preach the gospel to the world.
Here are links to related chapters or sections in Preaching the Gospel:
My Comments and Conclusion, Chapter 4