24:14 preached in all the world. Despite all the tribulations that would come—the deception of false teachers, the wars, persecutions, natural disasters, defections from Christ, and all the obstacles to the spread of the gospel—the message ultimately penetrates every part of the globe. God is never without a witness, and He will proclaim the gospel from heaven itself if necessary (cf. Rev. 14:6). and then the end will come. “The end” refers to the final, excruciating birth pangs (see note on v. 8). This is how Christ characterizes the time of Great Tribulation described in the verses that follow.
MacArthur, John Jr: The MacArthur Study Bible. electronic ed. Nashville : Word Pub., 1997, c1997, S. Mt 24:14

24:8 sorrows. The word means “birth pangs.” Famines, earthquakes, and conflicts have always characterized life in a fallen world; but by calling these things “the beginning” of labor pains, He indicated that things will get notably and remarkably worse at the end of the era as these unique tribulations signal the soon arrival of Messiah to judge sinful humanity and set up His millennial kingdom. Cf. 1 Thess. 5:3; Rev. 6:1–17; 8:1–9:21; 16:1–21;

MacArthur, John Jr: The MacArthur Study Bible. electronic ed. Nashville : Word Pub., 1997, c1997, S. Mt 24:8

 

And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come [Matt. 24:14].
The gospel of the kingdom is what John the Baptist preached—“Repent ye: for the kingdom of heaven is at hand” (Matt. 3:2). And the Lord Jesus began His ministry with that message—“From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand” (Matt. 4:17). Also, He sent His apostles out with that message (see Matt. 10). But in Matthew 11:28, we saw that our Lord’s message changed to “Come unto me, all ye that labour and are heavy laden, and I will give you rest.” And in Matthew 20:28 He said that He had come to give His life a ransom for many. But during the Tribulation Period the gospel of the kingdom will again be preached. It is not for our day, because we are to preach the gospel of the grace of God. Is the gospel of the kingdom another gospel? No, my friend, it is not. It is the same gospel with a different emphasis. We have no right to say that the kingdom of heaven is at hand because we don’t know. But when the Great Tribulation Period begins, the people will know that they are close to the end, although they will not know the day nor the hour. Therefore, the message will be, “Repent: for the kingdom of heaven is at hand.”
Now let me answer our critics who say that we who hold the dispensational view of Scripture teach that there are two or more ways of being saved. No, God has never had more than one basis on which He saves men, and that basis is the cross of Christ. Every offering before Christ came looked forward to the cross of Christ, and every commemoration since He has come looks back to the cross of Christ.
To illustrate this, let’s go back to Genesis 4 and look at the offering which Abel brought to God. He brought a little lamb. If you had been there, you could have asked Abel, “Why are you bringing this little lamb? Do you think that a little lamb will take away your sins?” He would have said, “Of course not! I’m bringing this little lamb because God told me to do so. I am bringing it by faith.” Then you could have asked him, “Well, if it won’t take away your sins, why would He ask you to bring it?” Abel’s answer would have been something like this: “This little lamb is pointing to One who is coming later, the seed of the woman, my mother. That One will take away our sins. I bring this little lamb by faith, recognizing that I am a sinner and need a substitute.” You see, Abel was looking forward to the One who was coming.
John the Baptist not only said, “Repent ye: for the kingdom of heaven is at hand” (Matt. 3:2), but he also said, “… Behold the Lamb of God, which taketh away the sin of the world” (John 1:29). John identified Him. Before the coming of Christ everyone who had come to God on His terms was saved on credit. And they were forgiven on the basis of the death of Christ. In the Old Testament God never saved anyone by Law. At the heart of the Mosaic system was the sacrificial system. They brought a lamb to God because the Law revealed that they were lawbreakers, that they were not obeying God, and that they did need to have a substitute to pay the penalty of their sins. The Law was given “… that every mouth may be stopped, and all the world may become guilty before God” (Rom. 3:19). My friend, you and I are lawbreakers, we are sinners needing a Savior. The thing to do is to receive Christ as your Savior before He comes as the Sovereign of this universe when He will be your Judge.
Now, going back to the verse we have been considering, “this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.” This does not mean that while the church is here in the world the end can’t come until the gospel of the grace of God is preached worldwide. I know there are those who use this verse to promote their Bible-teaching programs. While it is laudable to want to get the gospel to the ends of the earth, this is not the verse to use to promote it. You see, my friend, it is important to interpret Scripture in its context. Remember that our Lord is answering the question, “What is the sign of the end of the age?” (see v. 3). He is speaking of that end time.
McGee, J. Vernon: Thru the Bible Commentary. electronic ed. Nashville : Thomas Nelson, 1997, c1981, S. 4:127-128