Gregory the Dialogist

Hence also miracles were added to the holy preachers, so that the power displayed might give faith to their words, and that those who preached new things might do new things. When the world was flourishing, when the human race was increasing, when the flesh was subsisting long in this life, when the abundance of things was overflowing, who upon hearing would believe that there was another life? Who would prefer invisible things to visible things? …

Forty Gospel Homilies, Homily 4 ↗

Bonaventure

As regards the commission of authority for preaching the mysteries, he adds: And he sent them to preach the kingdom of God. For in the sending, authority is designated, without which no one ought to preach; hence Romans 10: "How shall they preach, unless they be sent?" Whence also Isaiah did not dare to preach unless he were sent; Isaiah 6: "Here am I, Lord, send me." On the contrary, concerning the false prophets, Jeremiah 23: "I did not send them, and they ran." But the Apostles were not such, who were so named precisely so that …

Commentary on Luke, Chapter 9 ↗