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.