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.
Note 7 at Mt 24:14: The "gospel of the kingdom" that Jesus referred to and what much of "Christianity" preaches today are not the same thing. The word "Gospel" means good news, or glad tidings. While it is certainly true that God hates sin and all sin will be judged, that is not the good news of the kingdom. Yet most religious preaching centers on what we do or should do and not on what Jesus has already done for us. The Gospel that Jesus referred to is that He has purchased a way to the Father through His own death and resurrection, and it is ours if we will just believe (Ro 10:1-13).
Many people who have heard the "bad news" of religion are working their fingers to the bone to earn God's favor. They have not only made Jesus their Savior, but they also have had the burden of salvation put on their own backs. That is not the Gospel of the kingdom. There are still millions of people who have not heard the good news that Jesus alone saves.
Note 8 at Mt 24:14: World evangelization is a must before the Lord will come back (compare with Mr 13:10). That is not to say that all people will believe on Jesus as their Savior, but every nation will hear the Gospel of the kingdom.