The proclamation of the Kingdom of God lies at the heart of Jesus' message, and it is a concept that permeates the Gospels. It is the lens through which the entirety of His ministry must be understood, a Kingdom that is both a present reality and a future hope, a reign of God that transforms both the individual and the world. For the Catholic Church, this Kingdom is not just a distant, abstract concept, but a lived reality that continues in the Church's mission and sacramental life. This book introduces the foundational theological and historical concepts of the Kingdom of God, offering a deep analysis of its significance in the preaching of Jesus. The concept of the Kingdom is deeply embedded in Jewish thought, and thus, understanding it in the context of Jesus' time requires a brief exploration of the history and expectations of the Jewish people. In the centuries preceding Jesus' birth, Israel had been under foreign dominion. From the Babylonians to the Persians, Greeks, and Romans, Israel's political and spiritual life had been significantly shaped by imperial forces. However, the hope for a Messianic Kingdom, a time when God would restore Israel to its rightful place and establish peace and justice for all nations, remained central to Jewish eschatology. This vision is captured in the Hebrew Scriptures, notably in texts such as the Prophets, Psalms, and in apocalyptic writings like Daniel. The Jewish people anticipated a Messiah who would come as a military leader, a king who would overthrow oppressors and restore the Kingdom to Israel.