By Saint Ambrose of Milan, Exposition of the Holy Gospel According to Saint Luke, translated by Theodosia Tomkinson, p. 24.
But those who are of God keep Faith and therefore cannot suffer from the moth which divides the garment. For everything which is divided within itself, like the kingdom of Satan, cannot be everlasting [cf. Mt. 11:25] Moreover, it is rust of the spirit when the keenness of religious intention is dulled by the defilement of worldly desires or the purity of faith is stained by a cloud of unbelief. Rust of the moth is desire for the familiar; rust of the mind is carelessness; rust of the mind is longing for honours, if the greatest hope of the present life is set thereon. And, therefore, let us run toward the Divine and let us sharpen our character; let us drill our disposition, so that we may have that sword which the Lord bade us sell our garment and buy [cf. Lk. 22:36], always ready and shining, as if sheathed in the scabbard of our mind. For the soldiers of Christ must always have strong spiritual weapons for the destruction of fortifications against God [cf. II Cor. 10:4], lest when He come, the Leader of the Heavenly Host [cf. Lk. 2:13], offended by the dullness of our weapons, separate us from the company of His legions.
from St. Ambrose, On the Mysteries and the Treatise on the Sacraments, Translations of Christian Liturature, Series III. Translated by T. Thompson, B.D., 1919, pp. 120-121.
Illustrations of the Blessings of Communion from Psalm 23 and from the Canticles
Therefore thou hast come to the altar, thou has received the body of Christ. Hear again what sacraments thou hast obtained. Hear holy David speaking. He too foresaw these mysteries in the spirit and rejoiced and said that he lacked nothing* Why? Because he that hath received the body of Christ shall never hunger.1
How often hast thou heard the twenty-second Psalm2 and not understood ? See how it is suited to the heavenly sacraments. The Lord is my shepherd; and I shall not want. In a green pasture, there hath he made me to lie down. He hath tended me by the water of comfort, he converteth my soul. He hath led me in the paths of righteousness for his name's sake. Yea, though I walk through the valley of the shadow of death, I will fear no evil, for thou art with me. Thy rod and thy staff, they have comforted me.3 Rod is rule, staff is passion ; that is the eternal Divinity of Christ, but also his passion in the body. The one created, the other redeemed. Thou hast prepared a table before me against them that trouble me. Thou hast anointed my head with oil, and my inebriating cup how glorious it is.4