1 Corinthians 9:2

2. yet doubtless--yet at least I am such to you. seal of mine apostleship--Your conversion by my preaching, accompanied with miracles ("the signs of an apostle," Ro 15:18, 19; 2Co 12:12), and your gifts conferred by me (1Co 1:7), vouch for the reality of my apostleship, just as a seal set to a document attests its genuineness (Joh 3:33; Ro 4:11).

Robert Jamieson, A. R. Fausset & David Brown, 1871 · public domain

Verse 2. If I be not an apostle unto others. "If I have not given evidence to others of my apostolic mission; of my being sent by the Lord Jesus, yet I have to you. Assuredly you, among whom I have laboured so long and so successfully, should not doubt that I am sent from the Lord. You have been well acquainted with me; you have witnessed my endowments, you have seen my success, and you have had abundant evidence that I have been sent on this great work. It is therefore strange in you to doubt my apostolic commission; and it is unkind in you so to construe my declining to accept your contributions and aid for my support, as if I were conscious that I was not entitled to that."

For the seal of mine apostleship; Your conversion is the demonstration that I am an apostle. Paul uses strong language. He does not mean to say that their conversion furnished some evidence that he was an apostle; but that it was absolute proof, and irrefragable demonstration, that he was an apostle. A seal is that which is affixed to a deed, or other instrument, to make it firm, secure, and indisputable. It is the proof or demonstration of the validity of the conveyance, or of the writing. Jn 3:33; Jn 6:27. The sense here is, therefore, that the conversion of the Corinthians was a certain demonstration that he was an apostle, and should be so regarded by them, and treated by them. It was such a proof,

(1.) because Paul claimed to be an apostle while among them, and God blessed and owned this claim.

(2.) Their conversion could not have been accomplished by man. It was the work of God. It was the evidence then which God gave to Paul and to them, that he was with him, and had sent him.

(3.) They knew him, had seen him, heard him, were acquainted with his doctrines and manner of life, and could bear testimony to what he was, and what he taught. We may remark, that the conversion of sinners is the best evidence to a minister that he is sent of God. The Divine blessing on his labours should cheer his heart, and lead him to believe that God has sent and that he approves him. And every minister should so live and labour, should so deny himself, that he may be able to appeal to the people among whom he labours, that he is a minister of the Lord Jesus.

Albert Barnes, 1834 · public domain

Verse 2 If I be not an apostle unto others - If there be other Churches which have been founded by other apostles; yet it is not so with you. The seal of mine apostleship are ye - Your conversion to Christianity is God's seal to my apostleship. Had not God sent me, I could not have profited your souls. The σφραγις or seal, was a figure cut in a stone, and that set in a ring, by which letters of credence and authority were stamped. The ancients, particularly the Greeks, excelled in this kind of engraving. The cabinets of the curious give ample proof of this; and the moderns contend in vain to rival the perfection of those ancient masters. In the Lord - The apostle shows that it was by the grace and influence of God alone that he was an apostle, and that they were converted to Christianity.

Adam Clarke, 1831 · public domain