So what is the Kingdom of Heaven?
"For he has rescued us from the dominion of darkness and brought us into the kingdom of the Son he loves, in whom we have redemption, the forgiveness of sins." Col 1:13 (NIV)
Although there are many nations, both past and present, there are really only two kingdoms. There is the kingdom of darkness (those who belong to Satan) and there is the Kingdom of God, (or Kingdom of Heaven) those who have been born again and belong to God.
Everyone on earth belongs to one or the other whether they do this knowingly or not.
In Colossians we read,
"For he has rescued us from the dominion of darkness and brought us into the kingdom of the Son he loves, in whom we have redemption, the forgiveness of sins." Col 1:13 (NIV)
Being saved is more than just sins washed away and a ticket to heaven. It's about a a change in allegiance. For lack of better words, it's membership in an entirely different nation. We become a people, united with every other believer on the face of the earth. Regardless of what language they speak, clothes they wear, or continent they live on.
And our kingdom is at war.
Paul famously changed sides when he went from killing Christians to preaching the Gospel. At the time of his conversion he asked Jesus, "Who are you, Lord?"
Jesus' reply clearly shows the change in allegiance and the Kingdom responsibility.
"'I am Jesus, whom you are persecuting,' the Lord replied. 'Now get up and stand on your feet. I have appeared to you to appoint you as a servant and as a witness of what you have seen and will see of me. I will rescue you from your own people and from the Gentiles. I am sending you to them to open their eyes and turn them from darkness to light, and from the power of Satan to God, so that they may receive forgiveness of sins and a place among those who are sanctified by faith in me.' Acts 26:15-18 (NIV)
What is the Kingdom of Heaven? Simply put, it is the family of God. Membership is automatic when we accept him as Lord.
