In Jesus’ parable, the seed of the farmer represents the Word of God. As this seed is scattered (Mark 4:3, Mark 4:14), it leads to a harvest (Mark 4:8, Mark 4:20). In like manner, as the Word of God is spread, it will lead to a harvest, the growth of the kingdom. The Word of God does the work of God.

After his death and resurrection and before his ascension into heaven, Jesus gave his disciples the task of being witnesses to his gospel (Matthew 28:18–20; Acts 1:6–11). They were given the task of sharing the truth about Jesus, of sharing his word. This is the way in which God’s kingdom is to grow: as his Word is being shared.

The Heidelberg Catechism summaries the Bible’s teaching in this regard when it asks the following two questions:

Question 83: What are the keys of the kingdom?
The preaching of the holy gospel and church discipline. By these two the kingdom of heaven is opened to believers and closed to unbelievers.

Question 84: How is the kingdom of heaven opened and closed by the preaching of the gospel?
According to the command of Christ, the kingdom of heaven is opened when it is proclaimed and publicly testified to each and every believers that God has really forgiven all their sins for the sake of Christ's merits, as often as they by true faith accept the promise of the gospel.
The kingdom of heaven is closed when it is proclaimed and publicly testified to all unbelievers and hypocrites that the wrath of God and eternal condemnation rest on them as long as they do not repent.

God’s judgment, both in this life and in the life to come, is based on this gospel testimony.

Since the kingdom of God grows as the Word of God is preached, this means that preaching the gospel is the primary task of God’s church. Not all Christians have been called to be gospel preachers but we have all been called to be witnesses of Jesus Christ with our words and our deeds (see Heidelberg Catechism, q/a 32 & 86). If we are not called to be ministers of the Word, we must support the preaching of the Gospel as we are able (see Heidelberg Catechism, q/a 103).

Since the kingdom grows as the Word of God is preached, this also means that we must not be distracted as a church community by various other tasks, no matter how beneficial they may be. Providing debt advice, a soup kitchen, child care, or other services to a community is not to be the focus of the church. These activities certainly have a place in the life of the Christian, and we can partner with other individuals in a private capacity to do them, but they are not the means by which the kingdom grows. It is not our service or love for our neighbour that grows the kingdom, but the preaching of the Word of God.