Mark 4:30, much like our parable from yesterday, deserves to be quoted as well in context:
Mark 4:30 Again he said, ?What shall we say the kingdom of God is like, or what parable shall we use to describe it? 31 It is like a mustard seed, which is the smallest seed you plant in the ground. 32 Yet when planted, it grows and becomes the largest of all garden plants, with such big branches that the birds of the air can perch in its shade.?
There have been two major interpretations here: one focusing on gradual growth and one on contrast. The first emphasizes the gradual growth of the kingdom until it takes over (often connected with postmillennialism) and the second the major contrast between present appearance and final glory. I prefer the second one. (I haven’t mentioned this book of mine: A New Vision for Israel; it is my textbook on the teachings of Jesus in historical context.)

1. The present manifestation of the kingdom is like a mustard seed and leaven: inauspicious, quiet, small.
2. The future manifestation of the kingdom is out of proportion to its present manifestation: thus, it will be magnificent and glorious and all pervasive.
3. Shade for birds probably refers to Gentile inclusion in kingdom (cf. Ezek 17:23; 31:6; Dan 4:10-12; Ps 104:12).
4. Work now, Jesus saying, in spite of how inconspicuous it looks because Pay Day is Coming!
5. Kingdom of God seems to refer here to the present humble state of God’s work among God’s People.
Here’s a tentative proposal: kingdom of God refers to the gospel, the good news of the inconspicuous underground/above ground work of God, in the present, among those who repent and believe in Jesus. These people “get it” (comprehend) and are restored and forgiven. This work of God is leading to a grand and final harvest-like judgment.
Again, see GR Beasley-Murray, Jesus and the Kingdom of God.