The "Kingdom of God" is mentioned throughout the NT.
John the Baptist preached it (Mt 3:1-2).
Jesus preached it (Mt 4:17).
After His resurrection, He spoke of it (Acts 1:3).
Paul preached it (Acts 28:31).
Hebrews sums it up in one phrase: "Therefore, since we are receiving a kingdom which cannot be shaken (12:28).
And lastly, Revelation summarizes it in one statement: "Now... the kingdom of our God... have come (12:10).
What would be a clear meaning of the word "kingdom" and a clear definition of the phrase "the kingdom of God?"