What does Luke 21:31 mean?

Ever since Jesus "set his face to go to Jerusalem" (Luke 9:51), He has been teaching His disciples about the kingdom of God. Then and now, the kingdom of God is any situation, place, or event where God's love, power, sovereignty, and authority shine through human fallenness. It is the motivating force in the hearts of His faithful followers. It is the spread of the gospel. But it is also the judgment of those who reject Him.

The kingdom of God will be fulfilled when Jesus returns, destroys Satan's forces, and takes the throne in Jerusalem. Despite what many "prophets" claim, we can't know when this is going to be at this time. Jesus specifically says, "It is not for you to know times or seasons that the Father has fixed by his own authority" (Acts 1:7).

What we know is that it will be at the end of the tribulation which Jesus describes in Luke 21:25–27:

"And there will be signs in sun and moon and stars, and on the earth distress of nations in perplexity because of the roaring of the sea and the waves, people fainting with fear and with foreboding of what is coming on the world. For the powers of the heavens will be shaken. And then they will see the Son of Man coming in a cloud with power and great glory."

He gives even more detail in the book of Revelation. When the events of the tribulation come, the kingdom of God will be near. Then, Jesus' full authority, which He defended on the temple Mount (Luke 20), will come.