After warning against divided service, the Lord now heals the anxiety that most often divides the heart. For men are not usually torn away from God by open wickedness, but by fear concerning the necessities of life. Thus He forbids anxious care, not labor itself, but the restless solicitude that distrusts divine providence.

When He says, “Take no thought,” He does not command idleness, but forbids that care which gnaws the heart and enslaves the mind. For there is a difference between providing what is needful and being consumed by fear about what may be lacking. The former belongs to human responsibility; the latter betrays a lack of faith.

The examples of the birds and the lilies are given to correct this inward disorder. God does not feed the birds without any activity on their part, yet He does so without anxiety. They do what is proper to their nature and leave the outcome to Him who governs all. Likewise, the beauty of the lilies shows that God can bestow what is necessary without human striving, lest man suppose that everything depends upon his own effort.

By reminding us that life is more than food and the body more than clothing, the Lord recalls the soul to what is greater. He who has given the greater will not withhold the lesser. To fear want is therefore to forget the generosity already shown by the Creator.

When He says that the Gentiles seek after these things, He does not condemn the use of temporal goods, but the manner of seeking them. Those who do not know God place their hope in such things; but those who know Him are called to seek first His kingdom and righteousness, trusting that what is necessary will be added in due measure.

The command to seek first does not exclude all concern for earthly needs, but orders it rightly. Temporal things are to be received as additions, not pursued as ends. When they are sought for their own sake, they burden the soul; when they are accepted as gifts, they serve it.

Finally, the Lord restrains anxiety about the future. Each day has its own burden, and to heap tomorrow’s cares upon today is to double one’s weakness. God gives strength for the present, not for imagined evils to come. Thus the soul is trained to live in trust, resting in divine providence from day to day, and freed from the anxious fear that would otherwise darken the heart.

Source: St. Augustine, On the Lord’s Sermon on the Mount,  Book II, Chapters 12-14