Paul wanted to give the Corinthians deeper teaching, but their behavior kept him on milk. Jealousy, strife, and factions revealed that they were thinking in merely human ways. Spiritual maturity is measured not by knowledge or gifts, which they had in abundance, but by love and unity. Their slogans about following Paul or Apollos showed they had not grown up.
Paul reframes the role of leaders. He and Apollos are servants through whom the Corinthians believed, each doing what the Lord assigned. In the agricultural image, Paul planted and Apollos watered, but God gave the increase. In the architectural image, Paul laid the foundation as a wise master builder, and others build on it. There can be no other foundation than Jesus Christ.
The building materials represent the quality of each person's ministry. Gold, silver, and costly stones last, while wood, hay, and straw do not. The Day will reveal each person's work, and enduring work will be rewarded. Paul then shifts from individual builders to the whole church: they are God's temple, and God's Spirit lives in them, so damaging the unity of the church is serious. Since all things belong to believers in Christ, and they belong to Christ, boasting in human leaders makes no sense.