The Lord now turns from the outward works of righteousness to the inward direction of the heart. For even when alms, prayer, and fasting are done in secret, the soul may still be divided if it clings to earthly treasure. Therefore He teaches that the place where one’s treasure is stored reveals where the heart truly rests.

Earthly goods are subject to decay and loss, not only because they perish in themselves, but because the heart that loves them is made anxious by their uncertainty. Heavenly treasure, by contrast, is secure, because it consists in the love of God and the hope of eternal life. When the heart is fixed there, it is not shaken by what passes away.

The single eye signifies a simple and upright intention. For as the eye guides the body, so intention governs the whole course of life. If intention is pure—directed toward God alone—then all actions, even those concerned with temporal things, are illumined. But if intention is corrupted by the desire for gain, praise, or security apart from God, the whole life is darkened.

This darkness is especially grievous because it disguises itself as light. For one may believe that one sees clearly, while in truth the very principle by which one judges is disordered. Thus the Lord warns that when intention itself is corrupt, no part of life remains untainted.

He therefore concludes by declaring the impossibility of divided service. To serve two masters is not merely difficult, but impossible, because each demands the whole heart. God requires love without reserve; mammon demands trust and attachment. The soul that tries to hold both is torn between them, and in the end despises one for the sake of the other.

By this teaching the Lord calls the heart to unity. The disciple must choose where to place his trust, his hope, and his love. When God is chosen as the sole treasure, the eye becomes single, the life becomes light, and the soul is freed from the bondage of divided desire.

Source: St. Augustine, On the Lord’s Sermon on the Mount,  Book II, Chapters 9-11