John Calvin 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. The purpose of this is to establish his authority among the Corinthians, placing it beyond all dispute. “If there are those,” he says, “who doubt my Apostleship, to you, at least, it ought to be beyond all doubt; for, since I planted your Church by my ministry, you are either not believers, or you must necessarily recognize me as an Apostle.”

And lest he seem to rest on mere words, he states that the reality itself was evident, because God had sealed his Apostleship by the faith of the Corinthians. Should anyone, however, object that this also suits false apostles, who gather disciples for themselves, I answer that pure doctrine is required above all things, so that one may have confirmation of his ministry in God’s sight from its effect.

Therefore, there is nothing here to give impostors reason for congratulation, if they have deceived any of the populace, indeed, even nations and kingdoms, by their falsehoods. Although in some cases people are instrumental in spreading the kingdom of Christ, who, nevertheless, do not preach the gospel sincerely, as is said in Philippians 1:16, Paul, with good reason, infers from the fruit of his labor that he is divinely commissioned. For the structure of the Corinthian Church was such that the blessing of God could easily be seen shining forth in it, which ought to have served as a confirmation of Paul’s office.