I Corinthians 9:2

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).

In this chapter the apostle seems to answer some cavils against himself. I. He asserts his apostolical mission and authority, and gives in his success among them as a testimony to it (Co1 9:1, Co1 9:2). II. He claims a right to subsist by his ministry, and defends it by several arguments from natural reason and the Mosaical law, and asserts it also to be a constitution of Christ (Co1 9:3-14). III. He shows that he had willingly waived this privilege and power for their benefit (Co1 9:15-18). IV. He specifies several other things, in which he had denied himself for the sake of other men's spiritual interest and salvation (Co1 9:19-23). And, V. Concludes his argument by showing what animated him to this course, even the prospect of an incorruptible crown (Co1 9:24 to the end.)