1 Corinthians 9:2

He had, 1 Corinthians 9:1, told them they were his work in the Lord, from whence he concludes here, that he was an apostle, that is, one sent of Christ to them for the good of their souls, whatever he was to others. You, saith he, as to yourselves at least, are the seal of my apostolical office; it hath a confirmation in you by the effect, as the writing is confirmed by the seal. For how can you think, that the blessing of the Lord should go along with my preaching, to turn you from pagan idolatry, and your lewd courses of life, to the true Christian religion, and to a holy life and conversation, if God had not send me. There is no such argument to prove a minister sent of Christ, as the success of his ministry in the conversion of souls unto God. It is true, we cannot conclude, that a minister is no true minister if he be able to produce no such seals of his calling; for the spiritual seed may for a time lie under the clods, and changes may be wrought in hearts, which are not published to the world; and even Isaiah may be sent to make the hearts of people fat. But where those seals can be produced, it is a most certain sign that the minister is a true minister, that is, one sent of God; for he could be no instrument to do such works if God were not with him; and if God had not sent him, he would not be with him so blessing his ministry. Yet it is possible the man may have his personal errors; for though some men doubt, whether an instance can be given of one openly and scandalously wicked, whom God ever honoured to be his instrument to convert souls, yet it would be rashly affirmed by any to say, that Judas (though a son of perdition, but not scandalous till the last) was an instrument to convert none.