John Gill, on 1Cor 9:2

If I be not an apostle unto others,.... This is said by way of supposition, not concession; for he was an apostle to many others; he was an apostle of the Gentiles in general; as the apostleship of the circumcision belonged to Peter, that of the uncircumcision fell to his share: but however, as if he should say, be that as it will, yet doubtless I am to you; all the signs of apostleship were wrought among them; not only the grace of God was implanted in them under his ministry, but the extraordinary gifts of the Spirit were received by them through it; and many signs, wonders, and mighty deeds, were done in the midst of them by him: see Co2 12:12 which were sufficient to put the matter quite out of doubt with them: for the seal of mine apostleship, are ye in the Lord; alluding to the sealing of deeds and writings, which render them authentic; or to the sealing of letters, confirming the truth of what is therein expressed; and the sense is, that their being converted persons, and so openly in the Lord, in union with him; or being made new creatures by the power of his grace, through his preaching, was an authentic proof of his apostleship, and served him instead of a letter testimonial and recommendatory; see Co2 3:1. Some copies read, "the seal of my epistle", and so the Ethiopic version.