Gregory the Great — on Ezek 3:9 (Homilies on Ezekiel, Book 1, Homily 10)

“But what is a sinner except one who is wounded? And what is a preacher except a physician? If therefore the sinner who lies in his wound is not ashamed, why should the physician be ashamed who provides healing through medicines? Often indeed it happens that a preacher is listened to reverently; but sometimes he is so despised by the perverse as if he were speaking nothing useful to them. Hence it is rightly said now: "I have made your face like adamant and like flint." Both diamond and flint are hard; but one of them is precious, the other worthless. The diamond is taken for adornment, the flint is trodden underfoot by travelers. And it often happens that when we observe those who receive our correction too humbly, we are ashamed to say certain things to them. But sometimes it happens that when we see those who disregard and hold in contempt our rebuke, we are afraid to bring them the word of preaching. But if we think rightly, we take up the authority of exhortation or rebuke both toward those by whom we observe ourselves to be honored, and toward those by whom we see ourselves to be despised, so that we ought neither to blush at the humility of the former, nor to fear the pride of the latter. Let it therefore be said: "I have made your face like diamond," that is, if you are honored by your hearers; "I have made your face like flint," if you are trampled upon and despised by your hearers, so that neither through honor conferred may the tongue be restrained by shame, nor through contempt may it be silent from weakness.”