Mark 4:26–29
26 And he said, “The kingdom of God is as if a man should scatter seed on the ground. 27 He sleeps and rises night and day, and the seed sprouts and grows; he knows not how. 28 The earth produces by itself, first the blade, then the ear, then the full grain in the ear. 29 But when the grain is ripe, at once he puts in the sickle, because the harvest has come.”
Jesus begins this parable the way he begins many of his parables, "The Kingdom of God is...". The Kingdom of God, or the Kingdom of Heaven, is Jesus' primary emphasis in his teaching. The implication is that through him God's Kingdom is coming more and more to the earth. The Kingdom of God can be summarized as God's people ruling under God's authority. Jesus' teaching inform his followers of it's characteristics, how to recognize it and even, as here, how it expands and grows.
The emphasis here is on the nature of the kingdom's growth. Again here, as in the parable of the soils, Jesus uses a simple seed analogy. Seeds don't grow by the ambition of the farmer or his constant labor. Instead once planted the farmer has nothing more to do but wait. He has no idea how, but after days of waiting, the seed will sprout and grow. "The earth produces by itself."
So Jesus is teaching that the advancement of the Kingdom of God comes not by the tireless labor of people, they only sow the seed and harvest it. Instead all the growth happens "by itself". Far from calling his followers to an uprising or a revolt, he calls them to simply preach the good news of the kingdom and allow God to grow it. They may not see it at the moment or understand how it's happening but the kingdom will continue to grow. Though Jesus was opposed by the religious leaders and misunderstood by much of the crowd following him, the kingdom, like a seed, was growing. They could trust in that and have confidence that God would continue causing it to grow.
This is the theology behind the whole book of Acts. Peter preaches a sermon at Pentecost and three thousand people believe! (Acts 2) The apostles are put in prison and an angel sets them free. (Acts 5:17-20) Jesus saves Paul on the road to Damascus. (Acts 9) Philip encounters on the road an Ethiopian who just happens to be reading a Messianic text in Isaiah. (Acts 8:26-40) Peter has a vision of unclean animals then gets a knock on the door to go see a Roman centurion at his house (he formerly wouldn't be permitted to enter his home because of the dietary cleanliness laws). (Acts 10) The whole book is full of events in which God moves behind the scenes, without the apostles knowledge or effort in some cases, and all they have to do is follow and reap the harvest.