Romans 10:15

Unless they be sent. Here is an evident proof against all new teachers, who have all usurped to themselves the ministry, without any lawful mission, derived by succession from the apostles, to whom Christ said, (John xx. 21.) As the Father hath sent me, I also send you. (Challoner) --- The Almighty sends people to preach two different ways. The one is extraordinary by internal inspiration, as was that of John the Baptist, and all the other prophets; in which case, however, extraordinary proofs must be given that they are sent by God; and the other is ordinary, which is derived from Christ, and from the apostles and their successors, whom he has appointed to be his vicegerents on earth. (Estius)