Which gospel does Matthew 24:14 mention?
Matthew 24:14 mentions "this gospel of the kingdom," not the gospel of grace. Jesus spoke from His earthly ministry, before His death and resurrection. At that time, Christ had not yet died or been raised, so the gospel of grace was not yet known.
Les explains that Jesus is speaking from within His earthly ministry time frame in Matthew 24. After verse 13 says the one who endures to the end will be saved, verse 14 says "this gospel of the kingdom." Many people twist that into the gospel of grace, and some object that this implies more than one gospel. Les says he has taught this for over twenty years. They did not know the gospel of grace at that time because Christ had not yet died and had not yet been raised from the dead.
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.