And how shall they preach, except they be sent? as it is written, How beautiful are the feet of them that preach the gospel of peace, and bring glad tidings of good things!
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Ambrosiaster
AD 400
Nobody can be a true apostle unless he is sent by Christ, nor will he be able to preach without a mandate to do so, for his testimony will not reflect his signs of power. Paul quotes the prophet Nahum. By talking about feet he means the coming of the apostles who went round the world preaching the coming of the kingdom of God. For their appearance enlightened mankind by showing them the way toward peace with God, which John the Baptist had come to prepare. This is the peace to which those who believe in Christ are hastening. Then St. Simeon, seeing the discord in the world, rejoiced at the coming of the Savior, saying: “Lord, now let your servant depart in peace,” because the kingdom of God is peace, and all discord is taken away when everyone bows the knee to the one God. Commentary on Paul’s Epistles.
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 my 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)
Paul is saying that by disbelieving, the Jews are not disbelieving the apostles only but also the prophet Isaiah, who foretold many years before that they would be sent to preach the gospel.
You have the work of the apostles also predicted: "How beautiful are the feet of them which preach the gospel of peace, which bring good tidings of good"
