St. Ambrose declares his desire that some angel would fly to him to purify him, as once the Seraph did to Isaiah— nay more, that Christ Himself would come to him, to the Emperor, and to his readers, and finally prays that Gratian and the rest of the faithful may be exalted by the power and spell of the Lord's Cup, which he describes in mystic language.
132. Howbeit, now must I needs confess the Prophet Isaiah's confession, which he makes before declaring the word of the Lord: “Woe is me, my heart is smitten, for I, a man of unclean lips, and living in the midst of a people of unclean lips, have seen the Lord of Sabaoth.” Now if Isaiah said “Woe is me,” who looked upon the Lord of Sabaoth, what shall I say of myself, who, being “a man of unclean lips,” am constrained to treat of the divine generation? How shall I break forth into speech of things whereof I am afraid, when David prays that a watch may be set over his mouth in the matter of things whereof he has knowledge? O that to me also one of the Seraphim would bring the burning coal from the celestial altar, taking it in the tongs of the two testaments, and with the fire thereof purge my unclean lips!
133. But forasmuch as then the Seraph came down in a vision to the Prophet, while You, O Lord, in revelation of the mystery have come to us in the flesh, do Thou, not by any deputy, nor by any messenger, but You Yourself cleanse my conscience from my secret sins, that I too, erstwhile unclean, but now by Your mercy made clean through faith, may sing in the words of David: “I will make music to You upon a harp, O God of Israel, my lips shall rejoice, in all my song to You, and so, too, shall my soul, whom You have redeemed.”
134. And so, O Lord, leaving them that slander and hate You, come unto us, sanctify the ears of our sovereign ruler, Gratian, and all besides into whose hands this little book shall come— and purge my ears, that no stains of the infidelity they have heard remain anywhere. Cleanse thoroughly, then, our ears, not with water of well, river, or rippling and purling brook, but with words cleansing like water, clearer than any water, and purer than any snow— even the words You have spoken— “Though your sins be as scarlet, I will make them white as snow.”
135. Moreover, there is a Cup, which You use to purify the hidden chambers of the soul, a Cup not of the old order, nor filled from a common Vine,— a new Cup, brought down from heaven to earth, filled with wine pressed from the wondrous cluster, which hung in fleshly form upon the tree of the Cross, even as the grape hangs upon the Vine. From this Cluster, then, is the Wine that makes glad the heart of man, uplifts the sorrowful, is fragrant with, pours into us, the ecstasy of faith, true devotion, and purity.
136. With this Wine, therefore, O Lord my God, cleanse the spiritual ears of our sovereign Emperor, to the end that, just as men, being uplifted with common wine, love rest and quietness, cast out the fear of death, have no feeling of injuries, seek not that which belongs to others, and forget their own; and so he, too, intoxicated with your wine, may love peace, and, confident in the exultation of faith, may never know the death of unbelief, and may display loving patience, have no part in other men's profanities, and hold the faith of more account even than kindred and children, as it is written: “Leave all that you have, and come, follow Me.”
137. With this Wine, also, Lord Jesus, purify our senses, that we may adore You, and worship You, the Creator of things visible and invisible. Truly, You can not fail of being Yourself invisible and good, Who hast given invisibility and goodness to the works of Your Hands.
Source: On the Christian Faith (De fide) (New Advent)