The Kingdom of Heaven is that nation of beings, both angelic and human, who hold Almighty God as Sovereign and who hold His laws and promises above those of any other nation or people, earthly or otherwise.
The citizens of the Kingdom of Heaven claim God as their God, and He, in turn, claims them as His people. This citizenship is a matter of fealty over geography. Citizens of the Kingdom of Heaven recognize and submit to the claim God rightfully has over their lives. Although as mortal humans they may not always submit to His Lordship in perfect obedience, love for Him drives them to pursue a deeper and more perfect knowledge of His will and His ways, seeking to please Him in all they do.
The foremost law of the Kingdom is to love the Lord God with all your heart, soul, mind and strength. The second is like it, to love your neighbor as yourself. Thus, the defining characteristic of a citizen of the Kingdom of Heaven is love that reflects the character of God in mercy, compassion, kindness, patience, goodness, faithfulness, righteousness, moral purity, humility, justice, and the like.
There will come a day when all citizens of the Kingdom of Heaven will dwell together in unity in the personal presence of God, but in this present age, only angelic citizens and those humans who have laid aside their mortal lives enjoy that privilege. Those of us who remain in our mortal bodies dwell as aliens and strangers on the earth, commissioned as ambassadors of the Kingdom to the citizens of the World.
As ambassadors, we represent the interests of the King of Kings and Lord of Lords within this world. We have been commissioned to participate in fulfilling His purpose, that is, to make disciples of all nations that as many as will might be rescued from the darkness, from enslavement to sin, that they might heed the call of Him who calls them out of darkness and into His marvelous light. Freedom from sin and darkness is to be realized here and now, even in the midst of a broken and hurting world. In Christ, we are blessed to enjoy the benefits of the Kingdom–freedom from the enslavement and guilt of sin, the peace and joy of the Spirit in the midst of trouble, the assurance of salvation, the privilege of His presence in prayer, the boundless love and grace of Christ–even while we wait in hope of the greater fulfillment of His Kingdom on earth.
As ambassadors, ours is a ministry of reconciliation. We bring good news to the enemies of the King that He Himself has provided a way for them to redeemed, to be reconciled. He offers full forgiveness and a chance to start fresh. He offers full citizenship in His Kingdom, with all the rights, privileges and responsibilities, both now and in the future when the Kingdom of this earth shall become the Kingdom of our God.
Ours is also a ministry of compassion, expressing the heart of God in the world as the hands and feet of Jesus: bringing a cup of cold clean water in His name, a warm blanket, a hot meal to the needy; a voice for those who cannot speak for themselves; standing for righteousness, justice and truth; providing a listening ear to the lonely; comfort to the brokenhearted; a gleam of hope to the hopeless, continuing the ministry of Jesus on earth until He returns.
At its heart, the Kingdom of Heaven is defined by its King. Its laws and principles are expressions of His nature. People become its citizens by virtue of their relationship to Him. Its mission in the world is the working out of His will. It is seen in the world through the working out of His power and presence.
The Kingdom of Heaven is an everlasting Kingdom whose King is Eternal, but the world is dying from sin and corruption. When the King Himself returns to re-claim His rightful place as Sovereign, He will subdue those remaining rebellious people who reject His rightful claim, and restore righteousness and justice upon the earth.