From the mustard seed we spoke above; now it is mulberry that must be treated. I read: a tree; I do not believe, however, that it is a tree: for what reason, what profit for us, that a tree, made to give fruit to the toilers who toil, is uprooted and thrown into the sea? Doubtless we believe it possible, by virtue of faith, that insensible nature obey perceptible orders; yet what does this species of tree mean? I read, it is true: "I was a shepherd of goats, grazing blackberries" (Amos 7:14), and I think the Prophet wanted to mark us that he was from the flock of sinners, sinner him himself, and withdrew from it. It is also fitting that, in order to prophesy to the nations, he sought fruits on the bushes, drew his food from the bushes. He was going to settle the Gentiles' dark and smelly flocks, the peoples of the nations, into the pastures of his writings, where they would fatten with spiritual nourishment, while he would draw from the sinner converted the spiritual milk. But, as in another book of the Gospels (Matt 17:19) is spoken of a mountain - whose bare silhouette, deprived of fertile vineyards and olive trees, barren in harvests, conducive to the dens of animals, troubled by the incursions of wild beasts, seems to translate the haughty elevation of the evil spirit (2 Cor 10:15), as it is written: "I speak to you, corrupt mountain, who corrupts the whole earth (Jer 51:25) - there is reason to believe that in this passage again we are shown faith expelling the foul spirit. Especially as the nature of the tree fits with this opinion: because its fruit is white at first in its flower, then once formed blushes, maturing becomes black. The devil too, fallen from his prevarication of the white flower of the angelic nature and the scarlet of his power, took the horrible blackness and odor of sin. See the One who says to the mulberry tree: "Rip yourself and throw yourself into the sea": when He hunts a Legion of a man, He allows him to pass into pigs, who, carried away by the diabolical impulse, have drowned in the sea (Luke 8:30 ff). This passage is therefore an exhortation to the faith; in the moral sense he teaches us that even what is most solid can be destroyed by faith. But from faith come charity, from charity hope, and they come back to one another as by a sacred circle.
The following shows that no one should boast of his works, since it is in justice that we owe the Lord our service. For if you do not say to a servant who has plowed or grazed the sheep, go on (here), put yourself at the table - where you hear that no one sits down if he does not pass first: Moses first began to move to see the great vision (Exod 3:3) - so if you not only do not say to your servant: sit down to table, but you claim from him another service and do not 'Do not thank it, so the Lord does not admit that you are giving Him a single work or work; for as long as we live, we must always work. Recognize, therefore, that you are a servant of many services. Do not worry about being called a child of God - you must recognize grace, but without forgetting nature - do not boast if you have served well: you had to do it. The sun does its work, the moon obeys, the angels do their service; the instrument chosen by the Lord for the Gentiles says, "I am not worthy to be called an apostle, because I have persecuted the church of God" (1 Cor 15:9); and in another place, after having shown that he is not aware of any fault, he adds: "But I am not justified for that reason" (1 Cor 4:4). So we, too, do not pretend to be praised for ourselves; do not anticipate the judgment of God; let us not prevent the judge's judgment, but reserve it for his time, for his judge.
Ambrose, Homilies on Luke 17