1Cor 9:2

“"If to others I am not an Apostle, yet at least I am to you." Do you see how far he is from enlarging here without necessity? And yet he had the whole world to speak of, and barbarous nations, and sea and land. However, he mentions none of these things, but carries his point by concession, and even granting more than he need. As if he had said, "Why need I dwell on things over and above, since these even alone are enough for my present purpose? I speak not, you will observe, of my achievements in other quarters, but of those which have you for witnesses. Upon which it follows that if from no other quarter, yet from you I have a right to receive. Nevertheless, from whom I had most right to receive, even you whose teacher I was, from those I received not." "If to others I am not an Apostle, yet at least I am to you." Again, he states his point by concession. For the whole world had him for its Apostle. "However," saith he, "I say not that, I am not contending nor disputing, but what concerns you I lay down. 'For the seal of mine Apostleship are ye:'" i.e., its proof. "Should any one, moreover, desire to learn whence I am an Apostle, you are the persons whom I bring forward: for all the signs of an Apostle have I exhibited among you, and not one have I failed in." As also he speaks in the Second Epistle, saying, "Though I am nothing, truly the signs of an Apostle were wrought among you in all patience, by signs and wonders and mighty works. For what is there wherein ye were made inferior to the rest of the Churches?" Wherefore he saith, "The seal of mine Apostleship are ye." "For I both exhibited miracles, and taught by word, and underwent dangers, and shewed forth a blameless life." And these topics you may see fully set forth by these two Epistles, how he lays before them the demonstration of each with all exactness.”

“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.”

“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.”

“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," Rom 15:18-19; Co2 12:12), and your gifts conferred by me (Co1 1:7), vouch for the reality of my apostleship, just as a seal set to a document attests its genuineness (Joh 3:33; Rom 4:11).”