In keeping with the goal of Mark’s Gospel, your sermon must answer the question, Who is Jesus?
Though Mark 4:1–20 does not explicitly say this, the kingship of Jesus Christ is relevant, and this passage thus teaches us that Jesus is the King who will have a kingdom harvest. The realization of who Jesus is (the King) and what he is doing (scattering his Word in order to produce a harvest) should encourage us to listen to him.
1 Again he began to teach beside the sea. And a very large crowd gathered about him, so that he got into a boat and sat in it on the sea, and the whole crowd was beside the sea on the land.