There are two types of parables: there are teaching parables (often using metaphors or similes) and there are kingdom parables. The kingdom parables usually begin with The kingdom of God/heaven...Jesus' words are about these kind of parables. The kingdom parables are "designed to capture the listener and make him a participant, overturning his world-view and leading him to call in question his most basic values." They teach the presentness of the kingdom and the immediacy of the kingdom of God. In the Old Testament in the Hebrew masal, they were an obscure saying or riddle. The crowds will hear the riddle, the hidden things and go home entertained - they hear and do not understand. Those who want to follow God will want to know what it means.
Speaking with the crowds, Jesus proclaims that the coming of the kingdom of heaven can be compared to that of a farmer who has completed his planting and now waits for the harvest.
Jesus leaves the mystery with the crowd. The disciples are also mystified. So Jesus explains the parable to them. The field is the world ( reminds me of Antony Gormley's Field) From the Early Church Fathers, through Augustine, and up to the Reformers, the field was seen as the church. Jesus is saying the field is the inhabited earth, the world, the kingdom is somewhere in it.
This parable reminds us that the fields are white for harvest. It is time to repent and believe. Kingdom parables are designed to captivate us and encourage us to challenge our deepest held world-view and our values.
The parable of the mustard seed follows. The seed becomes the mustard bush. This is what the kingdom of God is like. From something as small as a mustard seed something large grows that becomes the biggest of all kitchen herb and the birds of the air can perch in its shade, nest, settle, live, dwell, camp under its shade.
The parable has roots in Old Testament theology in Ps. 104:12, Ezk.17:23, when Israel will find shelter is at hand.
And so it is with us. Great events often have small beginnings. So we are encouraged to keep sowing however small the seed.