There were those who refused to call Paul an apostle and who did their best to subvert his message (see, e.g., Galatians 2:1–5; Galatians 6:12–15). Yet of all people, the Corinthians would surely have to acknowledge Paul’s apostleship.
In ancient times, a seal was used to prove the genuineness of a document. Likewise, the very existence of the Corinthian church proved the genuineness of Paul’s apostleship. It was through his apostolic ministry, a ministry accompanied by powerful demonstrations of the Spirit (1 Corinthians 3:4), that the church came into existence.
2 If to others I am not an apostle, at least I am to you, for you are the seal of my apostleship in the Lord.