The kingdom of God is as if a man should scatter seed on the ground, and should sleep by night and rise by day, and the seed should sprout and grow, he himself does not know how. For the earth yields crops by itself: first the blade, then the head, after that the full grain in the head. But when the grain ripens, immediately he puts in the sickle, because the harvest has come.
The parable is clearly about the process of growth, comparing the development of a plant from sowing to harvest to the spiritual maturation of a citizen of the Kingdom of God, a Christian. What sets this parable apart from other similar parables is that its emphasis is on the invisible and miraculous nature of growth. The sower may put the seed in the ground and do some cultivating, but "he himself does not know how" real development happens. God is behind the scenes, bringing His children to spiritual maturity in preparation for their harvest to eternal life.
Like its physical counterpart, spiritual growth happens slowly and incrementally. We should not expect a newly baptized Christian to be able to produce self-control as easily and to the same degree as one who has been in the church of God for several decades. In the parable, Jesus compares the Christian to a growing seed, and no one expects a sprout to produce ripe fruit immediately. This process takes time and steady progress through a series of stages of learning and experience.
This should be comforting, especially to those who are new in the faith. It should also set a goal or series of goals for each of us to strive toward. We do not want to remain a spiritual sprout like the wicked servant with one talent in the Parable of the Talents (Matthew 25:24-30). Fearful and lazy, he squandered all of his opportunities for growth by burying his talent while making excuses and blaming his master for his own shortcomings. Instead, we should desire to fulfill by the end of our spiritual lives the awesome goal Christ Himself gave us, to "be perfect, just as your Father in heaven is perfect" (Matthew 5:48).
As we continue to develop under Christ, we must accept that we may not see a great deal of improvement at any given time. It is not as some Christian pollsters try to tell us, that spiritual growth is easily quantifiable, whether we read so many chapters of the Bible or pray for x hours each day. Because it is of a spiritual nature—by definition, something that is beyond our physical senses—Christian growth can be difficult to determine, discernible only when a person's godly speech and actions reveal a marked improvement. We may be able to see growth somewhat crudely in certain stages, but most of it will occur unnoticed and unheralded.
The actual mechanics of spiritual growth are beyond understanding, like trying to fathom the infinite depths of the mind of God. As hymnist William Cowper wrote, "God moves in a mysterious way/His wonders to perform." We might as well ask how a kernel of grain becomes a fruitful stalk of wheat. All we really know is that God is faithful, continuing to work in His people to bring His crop of firstfruits to harvest (I Corinthians 1:4-9). He will make sure that every plant that He has chosen for His field has what it needs to grow, produce pleasing fruit, and enter into the fullness of His Kingdom.
The apostle Paul may have drawn upon this Parable of the Growing Seed in his analogy recorded for our edification in I Corinthians 3:6-9:
I planted, Apollos watered, but God gave the increase. So then neither he who plants is anything, nor he who waters, but God who gives the increase. Now he who plants and he who waters are one [united in their work], and each one will receive his own reward according to his own labor. For we are God's fellow workers; you are God's field, you are God's building.
He changes the metaphor at the end of the passage to God's building because he is progressing toward describing His people as the Temple of God (verses 16-17). However, the idea in his analogy is the same as in the parable, that God is the One who gives the increase to the crop in His field. He has provided the ministry to help things along (see Ephesians 4:11-16). He will give His sons and daughters whatever is needed to bring them to maturity—the best resources and experiences to cause real growth.
As he continues the building analogy, Paul cautions in I Corinthians 3:10, "But let each one take heed how he builds on it." He is speaking specifically to the ministry here, but this care also applies to the individual. In other words, returning to the growing-seed metaphor, the plant has some work to do too; it does not just stand in the soil and do nothing. Even though God provides the bulk of the resources for growth—water, nutrients, sunlight, etc.—the plant has to absorb them and use them to maintain itself and to grow.
In the illustration, the seed, activated by water, puts out roots and a shoot. Continued use of those resources causes it to put on height, develop a head, display flowers, and eventually produce fruit. God could spend eternity supplying sun and water to the earth, but if the seeds never responded to His blessings, not one sprout would ever break the soil's surface.
In the same way, God provides the knowledge, environment, energy, gifts, and whatever else is needed for a Christian to grow and produce spiritual fruit, but the Christian must consistently respond to God's providence to make them happen. Paul goes a step further and cautions us that, not only must each Christian respond, but he must be careful how he responds.
A plant that does not respond well to what God supplies withers and dies, and so does the improperly responsive Christian. Jesus addresses this in His Parable of the Sower (Matthew 13:1-9, 18-23), speaking of seed that "fell on stony places" and that "fell among thorns." These fail to grow due to "tribulation or persecution" or "the cares of this world and the deceitfulness of riches." In effect, they let external troubles and the stresses of living in this world halt the growth process. Such a Christian, Jesus says, "stumbles" or "becomes unfruitful."
Obviously, an analogy can be taken too far, but this one holds up well. God has planted us in His field, and He is looking for spiritual growth so that He can harvest us for His Kingdom. For our parts, we can cling to the promise in Malachi 4:2: "But to you who fear My name the Sun of Righteousness shall arise with healing in His wings; and you shall . . . grow. . . ."