What does Mark 4:26 mean?

The kingdom of God—here meaning the same thing as the kingdom of heaven—doesn't mean the end-times heaven or the afterlife, although it includes those places. It means any place or situation or moment where the sovereignty and power of God is evident. Justice, kindness, forgiveness, love—whenever qualities of God are on display, God's kingdom is present. So when Jesus says, "The kingdom of God is like…" He is describing a situation that both reflects God's character and glorifies Him.

All of the parables in Mark 4:21–34 elaborate on the parable of the sower (Mark 4:3–9), but the metaphors are decidedly mixed. In the parable of the sower, Jesus is the initial sower. In the parable of the growing seed, the sower is anyone who spreads the gospel. The parable of the sower describes how people respond to Jesus' teaching. The parable of the growing seed explains that the teacher's responsibility is to teach—to scatter seed. The teacher cannot force people to accept the message or to allow that message to make changes in their lives, any more than a farmer can force different types of soil to produce good crops.

Jesus' work with the twelve (Mark 3:13–19) is the foundation of what Paul tells Timothy in 2 Timothy 2:2: "…what you have heard from me in the presence of many witnesses entrust to faithful men, who will be able to teach others also." Paul tells the Corinthians, "So neither he who plants nor he who waters is anything, but only God who gives the growth" (1 Corinthians 3:7). Our job is to preach the truth; it is the work of the Holy Spirit to turn that truth into growth.