1 Corinthians 9:2

for the seal of mine apostleship are ye in the Lord If any Church had less right than another to question his Apostolic authority, it was the Church of Corinth, which he had founded (ch. 1 Corinthians 4:15), and on which so many spiritual gifts had been poured forth (ch. 1Co 1:5; 1 Corinthians 1:7, ch. 14). The Corinthians at least needed no other proof of the genuineness of his mission. "If any one wishes to know whether I am an Apostle, I will shew him yourselves; among whom are manifest and indubitable signs and proofs of my Apostolate; first the faith of Christ, which you have received at my preaching; then many and various gifts of the Holy Ghost." Estius. For the word seal see St John 3:33; John 6:27; Romans 4:11. A seal is used as the attestation of the genuineness of any document. Thus the existence of the Corinthian Church was the attestation of the genuineness of St Paul's Apostolic authority.