"Like a grain of mustard, which when sown on the earth, is the smallest of all seeds..." (Mark 4:31)
What is new about the good news? We read in Mark that Jesus proclaimed the good news of the Kingdom of God (Mark 1:14-15). What is new is that the time has come and the Kingdom is near.
How is this news good? It is God's Kingdom that is near--as opposed to the Kingdom of a human tyrant like Caesar or an angelic tyrant like Satan.
Who is king? It is the Father's kingdom, but the one serving as king is Jesus. Jesus says after his resurrection, "All authority in heaven and on earth has been given to me" (Matthew 28:18).
His kingdom is not like others. Jesus is at pains to explain these differences with parables that seem simple but are hard to understand.
One difference is that the Kingdom is not organized as a country, government, or institution. Rather, the Kingdom is a seed sown in the soil of a human heart.
As such, it begins tiny, like a mustard seed, and grows large, like a mustard plant. In neither case is the Kingdom overwhelming in size or power. It is manageable enough that an ordinary person can meaningfully relate to it. We can nurture the Kingdom just as we can water a seed. And we can make it grow like we grow a plant.
Jesus says the Kingdom will come in power--which sounds dramatic--but this power turns out to be the power that a leafy shrub has to provide protective shade to the birds of the air.
Jesus teaches in order to plant this seed in the good soil of our hearts, where it might be met with understanding and joy and where it might bear fruit.