In Mark's Gospel, it is proclaimed that, "the time is fulfilled, and the kingdom of God has come near; repent, and believe in the good news." This line from Mark 1:15 demonstrates an important part of Jesus' ministry. Jesus held the idea that Israel was meant for fulfillment, and therefore, because the kingdom of God has arrived, this time has finally been reached. God posseses a divine plan in bringing about salvation, right now and in the future. It speaks not of the past, but simply of a divine plan for the healing and liberation of all people, both in present and future. Or, referred to in written theology as 'already and not yet.'
Jesus is instructing those who have faith in and trust God to believe that indeed the good news is upon them. Repentance, for Jesus, meant changing one's entire worldview. He is bringing about a revolutionary large scale change in popular belief, lifestyle, and customs - deeply rooted in strong faith in the master plan at hand. Jesus never formally defines the kingdom of God. It is, more or less, an intangible idea akin to love or beauty. It is also present in taking action and by following Jesus' teachings of establishing peace and justice, well-being and salvation for all humanity.