This is today's Gospel reading from Luke 13: Jesus said, "What is the kingdom of God like? To what can I compare it? It is like a mustard seed that a man took and planted in the garden. When it was fully grown, it became a large bush and the birds of the sky dwelt in its branches." Again he said, "To what shall I compare the kingdom of God? It is like yeast that a woman took and mixed in with three measures of wheat flour until the whole batch of dough was leavened."
This tells me that the kingdom of God is to let God completely fill us so that we bear fruit to the max. This is to be in union with him. When we let God do whatever he wishes with us, we are free! And the kingdom of God now belongs to us.