1 Corinthians 9:2

Those Jewish believers who nevertheless continued to observe the law of Moses denied that Paul was an apostle because he taught that it was no longer necessary to be circumcised or to observe the sabbath. Even the other apostles thought that he was teaching something different because of this, and they denied that he was an apostle. But to the Corinthians Paul was an apostle, because they had seen the signs of God's power in him.

"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.