The story of the Kingdom of God is an ancient one. In the dominion of Adam over all created things (cf. Gen 1:26-28), in the promise to Abraham that kings would be among his descendants (cf. Gen 17:6), in the promise that the “scepter” and the “ruler’s staff” would come to Judah (cf. Gen 49:8-10), in the promise given to the Hebrew people at Mt. Sinai that God would make them “a kingdom of priests and a holy nation” (Exo 19:5-6), in the reign of King David and the promise to him of an everlasting kingdom (cf. 2 Sam 7:16; Psa 89:35-36), and in the words of the prophets after the Babylonian Captivity (cf. Isa 9:5-6; Jer 23:5; Ezek 34:23; Zech 9:9; 14:9) -- in all this we see that it was the plan of God from the very beginning to call a People unto Himself and reign over them as King.
Jesus Christ came to bring that plan to fruition. Indeed, when the angel Gabriel came to Mary to prepare for the coming of the Lord through her, he said of her Son that “the Lord God will give to him the throne of his father David, and he will reign over the house of Jacob for ever; and of his kingdom there will be no end" (Lk 1:32-33). Jesus is the fulfillment of all of the prophecies and promises of God about the Kingdom He would establish. But, what is this kingdom and how can we be a part of it?
The “Kingdom of God” can really be understood in three ways. It is Christ Himself, it is His rule in the hearts of the faithful, and it is the Church.
It may be difficult to understand how Jesus IS the kingdom. But, if the kingdom is wherever God reigns, and God reigns on earth through Christ, then wherever Christ is, there is the kingdom. Jesus Himself was clear on this point: “If it is by the finger of God that I cast out demons, then the kingdom of God has come upon you” (Lk 11:20), and again, “Behold, the kingdom of God is in your midst” (Lk 17:21). Of course, it follows from this that, if “Christ dwells in your hearts through faith” (Eph 3:17) then the Kingdom of God is also in the hearts of men. Finally, the Church is the Kingdom insofar as it is through the Church that Jesus exercises his power and authority over the People of God on earth. Jesus gave to Peter “the keys of the kingdom of heaven” (Mt 16:19) and gave the apostles a share in this authority (cf. Mt 18:18; Lk 22:29-30). In Heb 12 we see that the "assembly (or 'church') of the firstborn" (vs. 23) is the "kingdom that cannot be shaken" (vs. 28).
We receive the grace that grants us entrance into the Kingdom through the sacraments of the Church (cf. Jn 3:5) and we experience His benevolent rule over our lives through Her ministers. It is in fact through the Church that we experience the Kingdom of God to the fullest, until that day comes when Jesus will bring all things under His power and will Himself be all in all (cf. 1 Cor 15:28; Eph 1:23).