What does Mark 4:28 mean?

The parable of the growing seed explains that God calls teachers to teach, but no person can actually bring about spiritual maturity in another. The Holy Spirit causes spiritual growth. The "full grain in the ear" represents a believer who has grown in faith enough to spread the gospel to others.

It's tempting to make a theology of salvation out of the progression of blade, ear, and full grain, but that is not Jesus' intent in this instance. Still, there are some lessons here relevant to salvation. Jesus talks about grain that germinates and grows into stalks but is destroyed by the sun or the thorns (Mark 4:5–7). This represents people who hear the gospel and react favorably at first, but can't withstand the trials, persecution, or their love of worldly riches (Mark 4:16–19).

Conversely, authentic salvation always results in visibly changed lives (James 2:17). As we grow in faith, the Holy Spirit changes us, allowing us to express His character (Galatians 5:22–23). Critically, though, we should remember that just as the Holy Spirit gives believers different gifts (Romans 12:6–8; 1 Corinthians 12:4–11), each individual grows in a different way. We can't judge the salvation of another who doesn't grow the way we expect them to (Romans 14:1–4).

The most important growth, one every believer should exhibit, is the desire to spread Jesus' gospel to others. This is why He chooses the twelve (Mark 3:13–15). After His resurrection, He gives the twelve the Great Commission to spread the gospel, make disciples, baptize new believers, and teach them to obey His words (Matthew 28:18–20). That command is for us, as well.