1 Cor. 9:1-2 (ESV)
Am I not free? Am I not an apostle? Have I not seen Jesus our Lord? Are not you my workmanship in the Lord?  If to others I am not an apostle, at least I am to you, for you are the seal of my apostleship in the Lord.
Paul starts off chapter 9 attacking those who question his apostleship. Paul spends a lot of time in his letters doing this. He has to. If he is not an apostle than his authority to teach what he taught is undermined, and there is no reason to believe him. An apostle is someone sent with the authority of another, in this case, Christ. At the time of Paul, one could sent an “apostle” and it was as if the one who sent the apostle was there in person. An apostle could even marry on behalf of the one who sent him. (Doubtful he was privileged to the conjugal rights, but…) So when Paul spoke as an apostle, it wasn’t him speaking, but Christ. He could not afford to let his apostleship be questioned. The authority of the gospel rested on it. Similarly, pastors should not allow their call to be questioned. It is only by the authority of Christ given to us in our call and ordination that we can forgive sins. That is the primary job of the pastor, to forgive sins. When there is reason to doubt a pastor’s credentials then the gospel the pastor preaches is threatened.
However, Paul’s credentials were somewhat weak it seemed. He was not one of the twelve. He had not followed Jesus during his earthly ministry. Other apostles like Peter were less questionable. But then when Paul had to call Peter on the carpet for acquiescing to the false apostles and Judaizers who wanted to put the gentile Christians back under the law with circumcision (Gal. 2), Peter did not even attempt to question Paul’s apostleship. There was no, “who do you think you are?” coming from the lips of Peter.
But here Paul just lets it be not if to others I am not an apostle I don’t care. I am to you, you are my workmanship, the seal of my apostleship. Indeed most of us are the workmanship of Paul. It is doubtful any of us would be Christian without his recorded works in the Bible. Paul’s apostleship has never died.