The Lord now removes the desire for retaliation from the heart. For the Law restrained vengeance by limiting it, so that injury might not exceed injury; but Christ uproots vengeance altogether, teaching the soul to bear wrong patiently. The former rule measured retribution; the latter extinguishes it by charity.

This command is not given so that evil may be loved, but that the heart may not be conquered by it. For to resist evil in the manner of retaliation is to let another’s sin draw one into sin. By bearing injury willingly, the disciple preserves interior freedom and does not allow injustice to produce hatred. Thus the evil is overcome, not by force, but by goodness.

The turning of the other cheek does not signify a rigid outward act required in every circumstance, but an inward readiness of spirit. The soul must be prepared to endure further injury rather than seek revenge. Likewise, yielding one’s cloak or walking the additional mile signifies a heart loosened from attachment to possessions and dignity, preferring peace and charity to self-assertion.

These examples are not meant to multiply injuries, but to heal the will. For when one willingly yields more than is demanded, the aggressor is deprived of the satisfaction of contention, and the injured person remains uncorrupted by anger. What is surrendered outwardly preserves what is greater inwardly.

The command to give to those who ask is governed by wisdom and charity, not blind indulgence. The heart must be generous and free from hardness, even when prudence guides the act. Thus the Lord forms a soul that does not cling to its rights, but entrusts itself to God, who alone judges justly.

In this way, righteousness advances beyond the balance of strict justice into the fullness of mercy. The disciple no longer asks what is permitted by law, but what is fitting for love, learning to suffer wrong without bitterness and to imitate the patience of God Himself.

Source: St. Augustine, On the Lord’s Sermon on the Mount,  Book I, Chapters 15-17