The prophet is silent because, at that moment, it is not he who speaks, but another. God says to his prophets, poor sinful human creatures, “You shall be as my own mouth” (cf. Jeremiah 15:19), and the thought of it makes his messenger tremble.
Of course, this doesn’t happen at the same level of intensity all the way through; there are special moments. God needs only one phrase, one word. The speaker and the listeners have the feeling that drops of fire mingle at a certain point with the preacher’s words and they become white-hot and shining. Of all images, fire is the one that is least inadequate when it comes to expressing this operation of the Spirit.
— “Faith Which Overcomes the World” by Fr. Raniero Cantalamessa