From a letter by Saint Ambrose, bishop (Epist. 2, 1-2. 4-5. 7: PL [edit. 1845,] 879, 881)
By the grace of your words win over your people
You have entered upon the office of bishop. Sitting at the helm of the Church, you pilot the ship against the waves. Take firm hold of the rudder of faith so that the severe storms of this world cannot disturb you. The sea is mighty and vast, but do not be afraid, for as Scripture says: he has founded it upon the seas, and established it upon the waters.
The Church of the Lord is built upon the rock of the apostles among so many dangers in the world; it therefore remains unmoved. The Church's foundation is unshakeable and firm against the assaults of the raging sea. Waves lash at the Church but do not shatter it. Although the elements of this world constantly beat upon the Church with crashing sounds, the Church possesses the safest harbor of salvation for all in distress. Although the Church is tossed about on the sea, it rides easily on rivers, especially those rivers that Scripture speaks of: The rivers have lifted up their voice. These are the rivers flowing from the heart of the man who is given drink by Christ and who receives from the Spirit of God. When these rivers overflow with the grace of the Spirit, they lift up their voice.
There is also a stream which flows down on God's saints like a torrent. There is also a rushing river giving joy to the heart that is at peace and makes for peace. Whoever has received from the fullness of this river, like John the Evangelist, like Peter and Paul, lifts up his voice. Just as the apostles lifted up their voices and preached the Gospel throughout the world, so those who drink these waters begin to preach the good news of the Lord Jesus.
Drink, then, from Christ, so that your voice may also be heard. Store up in your mind the water that is Christ, the water that praises the Lord. Store up water from many sources, the water that rains down from the clouds of prophecy.
Whoever gathers water from the mountains and leads it to himself or draws it from springs, is himself a source of dew like the clouds. Fill your soul, then, with this water, so that your land may not be dry, but watered by your own springs.
He who reads much and understands much, receives his fill. He who is full, refreshes others. So Scripture says: If the clouds are full, they will pour rain upon the earth.
Therefore, let your words be rivers, clean and limpid, so that in your exhortations you may charm the ears of your people. And by the grace of your words win them over to follow your leadership. Let your sermons be full of understanding. Solomon says: The weapons of the understanding are the lips of the wise; and in another place he says: Let your lips be bound with wisdom. That is, let the meaning of your words shine forth, let understanding blaze out. See that your addresses and expositions do not need to invoke the authority of others, but let your words be their own defense. Let no word escape your lips in vain or be uttered without depth of meaning.