5 The mystery of the holy Church is manifest in its very foundation. The Lord Jesus set it on its course by preaching the Good News, that is, the coming of the Kingdom of God, which, for centuries, had been promised in the Scriptures: "The time is fulfilled, and the kingdom of God is at hand". In the word, in the works, and in the presence of Christ, this kingdom was clearly open to the view of men. The Word of the Lord is compared to a seed which is sown in a field; those who hear the Word with faith and become part of the little flock of Christ, have received the Kingdom itself. Then, by its own power the seed sprouts and grows until harvest time. The Miracles of Jesus also confirm that the Kingdom has already arrived on earth: "If I cast out devils by the finger of God, then the kingdom of God has come upon you". Before all things, however, the Kingdom is clearly visible in the very Person of Christ, the Son of God and the Son of Man, who came "to serve and to give His life as a ransom for many:" When Jesus, who had suffered the death of the cross for mankind, had risen, He appeared as the one constituted as Lord, Christ and eternal Priest, and He poured out on His disciples the Spirit promised by the Father. From this source the Church, equipped with the gifts of its Founder and faithfully guarding His precepts of charity, humility and self-sacrifice, receives the mission to proclaim and to spread among all peoples the Kingdom of Christ and of God and to be, on earth, the initial budding forth of that kingdom. While it slowly grows, the Church strains toward the completed Kingdom and, with all its strength, hopes and desires to be united in glory with its King.
Source: Lumen Gentium (Vatican.va)