49 “Therefore,” says He, “I say unto you, Have not anxiety for your life, what you shall eat; nor yet for your body, what you shall put on.” Lest perchance, although it is not now superfluities that are sought after, the heart should be made double by reason of necessaries themselves, and the aim should be wrenched aside to seek after those things of our own, when we are doing something as it were from compassion; i.e. so that when we wish to appear to be consulting for some one's good, we are in that matter looking after our own profit rather than his advantage: and we do not seem to ourselves to be sinning for this reason, that it is not superfluities, but necessaries, which we wish to obtain. But the Lord admonishes us that we should remember that God, when He made and compounded us of body and soul, gave us much more than food and clothing, through care for which He would not have us make our hearts double. “Is not,” says He, “the soul more than the meat?” So that you are to understand that He who gave the soul will much more easily give meat. “And the body than the raiment,” i.e. is more than raiment: so that similarly you are to understand, that He who gave the body will much more easily give raiment.
50. And in this passage the question is wont to be raised, whether the food spoken of has reference to the soul, since the soul is incorporeal, and the food in question is corporeal food. But let us admit that the soul in this passage stands for the present life, whose support is that corporeal nourishment. In accordance with this signification we have also that statement: “He that loves his soul shall lose it.” And here, unless we understand the expression of this present life, which we ought to lose for the kingdom of God, as it is clear the martyrs were able to do, this precept will be in contradiction to that sentence where it is said: “What is a man profited, if he shall gain the whole world, and lose his own soul?”
51. “Behold,” says He, “the fowls of the air: for they sow not, neither do they reap, nor gather into barns; yet your heavenly Father feeds them: are you not much better than they?” i.e. you are of more value. For surely a rational being such as man has a higher rank in the nature of things than irrational ones, such as birds. “Which of you, by taking thought, can add one cubit unto his stature? And why do you take thought for raiment?” That is to say, the providence of Him by whose power and sovereignty it has come about that your body was brought up to its present stature, can also clothe you; but that it is not by your care that it has come about that your body should arrive at this stature, may be understood from this circumstance, that if you should take thought, and should wish to add one cubit to this stature, you cannot. Leave, therefore, the care of protecting the body to Him by whose care you see it has come about that you have a body of such a stature.
52. But an example was to be given for the clothing too, just as one is given for the food. Hence He goes on to say, “Consider the lilies of the field, how they grow; they toil not, neither do they spin: and yet I say unto you, that even Solomon in all his glory was not arrayed like one of these. Wherefore, if God so clothe the grass of the field, which today is, and tomorrow is cast into the oven; shall He not much more clothe you, O you of little faith?” But these examples are not to be treated as allegories, so that we should inquire what the fowls of heaven or the lilies of the field mean: for they stand here, in order that from smaller matters we may be persuaded respecting greater ones; just as is the case in regard to the judge who neither feared God nor regarded man, and yet yielded to the widow who often importuned him to consider her case, not from piety or humanity, but that he might be saved annoyance. For that unjust judge does not in any way allegorically represent the person of God; but yet as to how far God, who is good and just, cares for those who supplicate Him, our Lord wished the inference to be drawn from this circumstance, that not even an unjust man can despise those who assail him with unceasing petitions, even were his motive merely to avoid annoyance.
Source: Our Lord's Sermon on the Mount (New Advent)