What does 1 Corinthians 3:9 mean?

Paul concludes his agricultural metaphor and begins a new one within the space of a few words in this verse. He is showing the Corinthians their divisive loyalty to one Christian leader over another is misplaced. Paul wants them to be unified, and not divided, so he is showing how pointless their bickering really is.

Instead of being important pillars of the movement of the Christian church, Paul has described the work he and Apollos do as it really is. They serve in God's field, with one spreading the seed and the other following with the water bucket. They are co-workers, each performing a necessary task. They are also only common laborers doing the dirty work of establishing a crop. They might have different styles, or unique approach, but they are in fundamental agreement about the gospel. There is no reason for people to align themselves under the banner of Paul, or Apollos, or Peter, in opposition to others who "follow" some different teacher.

In short, God is the one who grows the crops, and they both work for Him. Paul is saying, "Don't follow the field workers; follow the owner of the field." Paul describes the Corinthians as the field itself. They are the ones in which the gospel was planted. God is the one who caused that gospel to take root and grow in them, bringing them to faith in Christ and eternal life.

Paul immediately also calls the Corinthians God's building or construction project, introducing a new metaphor he will build on in the following verses.