When will the gospel of the kingdom be preached in all the world?
At a future time, during the tribulation, the gospel of the kingdom will be preached in all the world as a witness to all nations. This is the same kingdom gospel Jesus and the Twelve proclaimed, and it will be proclaimed again before the seven-year tribulation ends and the end comes.
Les explains that Jesus was using the backdrop of the kingdom gospel He and the Twelve had been preaching for almost three years. The verb is future tense: this gospel of the kingdom shall at a future time be preached in all the world for a witness unto all nations, and then shall the end come. Many lift this verse out of context and say the gospel must go into every nation before the Lord can come, but that is not what Jesus was talking about. The gospel of grace calls out a people for His name, but it will end. The tribulation will come, and then the same gospel of the kingdom that Jesus preached in His three years of ministry will be proclaimed again during the tribulation. If that good news is proclaimed to every nation on earth, the King is coming in just seven years from day one.