2 But why should I confine my allusions to the men of this world, when the Apostle Paul, the chosen vessel the doctor of the Gentiles, who could boldly say: “Do ye seek a proof of Christ speaking in me?” knowing that he really had within him that greatest of guests— when even he after visiting Damascus and Arabia “went up to Jerusalem to see Peter and abode with him fifteen days.” For he who was to be a preacher to the Gentiles had to be instructed in the mystical numbers seven and eight. And again fourteen years after he took Barnabas and Titus and communicated his gospel to the apostles lest by any means he should have run or had run in vain. Spoken words possess an indefinable hidden power, and teaching that passed directly from the mouth of the speaker into the ears of the disciples is more impressive than any other. When the speech of Demosthenes against Æschines was recited before the latter during his exile at Rhodes, amid all the admiration and applause he sighed “if you could but have heard the brute deliver his own periods!”
Source: Letters (New Advent)