Setting: One of the lakeside kingdom-parables of Mark 4, taught "by the sea" in parables (Mark 4:1-2). It sits between the [Sower] (the word and the soils) and the [Mustard Seed] (small beginning, great end), and is found only in Mark, the one parable unique to that gospel. Citation: Mark 4:26-29
The parable
"God's Kingdom is as if a man should cast seed on the earth, and should sleep and rise night and day, and the seed should spring up and grow, though he doesn't know how. For the earth bears fruit by itself: first the blade, then the ear, then the full grain in the ear. But when the fruit is ripe, immediately he puts in the sickle, because the harvest has come." (vv.26-29).
The point: the kingdom grows by God's own power
The farmer's part is small and bracketed at both ends, he scatters the seed, and at the end he reaps. In between he simply "should sleep and rise night and day": he cannot make the seed grow, and he does not even understand the process ("he doesn't know how"). The growth is the earth's doing, the Greek says the soil bears fruit αὐτομάτη (automatē), "of itself," "automatically", that is, by a power not the farmer's.
This is a parable of trust and patience for disciples impatient to see the Kingdom arrive. The Kingdom of God advances not by human effort, agitation, or force but by God's own life working invisibly, in its own time, through stages that cannot be rushed (blade → ear → full grain). The harvest is certain, and it is God who brings it.
The closing line, "he puts in the sickle, because the harvest has come", quotes Joe.3.13, an image of the final ingathering (echoed again in Revelation 14:15). So the parable also has an eschatological horizon: the secret growth now will end in a decisive harvest then.
Theological framing (multiple views)
Reassurance reading (mainstream), comfort for disciples who see slow, hidden beginnings: God guarantees the growth and the harvest; the worker can rest in that, not strive anxiously.
Anti-zealot / non-coercion reading, the Kingdom does not come by political force or human engineering ("the earth bears fruit by itself"); a quiet rebuke to those who would seize it (cf. John 6:15).
Eschatological-harvest reading (e.g. C. H. Dodd, Jeremias), the weight falls on the certain harvest: the present hidden growth is the in-breaking Kingdom moving surely toward its consummation/judgment.
Process / sanctification application, read pastorally, a picture of how grace grows in a life and in the church, gradually, organically, not by force, and a caution against both passivity and over-management.
All readings agree on the central contrast: the growth is God's, not the sower's.