1 Corinthians 9:1 directly precedes, asking 'Are you not my workmanship?' — the same idea of them as proof of his apostleship.

1 Corinthians 4:15 calls Paul their father in Christ through the gospel, the very relationship that makes them the seal of his apostleship.

1 Corinthians 3:10 describes Paul as master builder laying the foundation at Corinth, explaining how they became his apostolic seal.

2 Corinthians 3:1 asks about letters of recommendation—Paul's point: the Corinthians themselves are his seal of apostleship (9:2).

In 2 Corinthians 12:12, Paul lists the signs of a true apostle performed among the Corinthians, confirming the authenticity he claims here.

2 Corinthians 3:2 calls the Corinthians Paul's letter of recommendation, a parallel image to them being the seal of his apostleship.

Galatians 2:8 affirms Paul's apostleship to the Gentiles, paralleling his claim that the Corinthians are the seal of his apostleship.