Albert Barnes Commentary 1 Corinthians 9:2

"If to others I am not an apostle, yet at least I am to you; for the seal of mine apostleship are ye in the Lord." — 1 Corinthians 9:2 (ASV)

If I am not an apostle to 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 labored 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 that you doubt my apostolic commission, and it is unkind of you to interpret 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 my apostleship; your conversion is the demonstration that I am an apostle. Paul uses strong language. He does not mean to say that their conversion merely provided some evidence that he was an apostle, but that it was absolute proof and irrefutable demonstration that he was. 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 (John 3:33; John 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 and treated by them. It was such a proof for these reasons:

1. Because Paul claimed to be an apostle while among them, and God blessed and confirmed 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 from God. The divine blessing on his labors should cheer his heart and lead him to believe that God has sent him and that He approves of him. And every minister should so live and labor, should so deny himself, that he may be able to appeal to the people among whom he labors that he is a minister of the Lord Jesus.