The Lord here gathers into a brief and comprehensive rule all that has been taught concerning human conduct. For whatever one desires to receive from another, one should first practice toward that other. This precept does not merely restrain injustice, but calls forth charity, since it urges the soul to consider the good of another from the measure of its own natural love of self.

This rule does not require subtle reasoning, for each man carries within himself a witness to what is fitting. One need not inquire endlessly what is owed to another; one has only to consult one’s own heart. In this way the Lord places the law of love close at hand, making it accessible to all who are willing to judge themselves honestly.

By adding that this is the Law and the Prophets, He teaches that all commandments concerning our neighbor are fulfilled in charity. Whatever the Law enjoined and the Prophets proclaimed regarding justice, mercy, and faithfulness is contained within this single principle. Thus the multiplicity of precepts is reduced to unity, not by abolishing them, but by revealing their common end.

This rule also presupposes what has gone before. One who judges humbly, prays faithfully, trusts in God, and loves even enemies is now able to live this command sincerely. Without such interior formation, the rule could be twisted to serve self-interest; but when the heart is purified, it becomes a true measure of charity.

Therefore the Lord places this saying at the close of His moral teaching, as a seal upon what has been spoken. For when this rule is kept in truth, nothing contrary to love is done, and the soul walks safely in the fulfillment of the Law.

Source: St. Augustine, On the Lord’s Sermon on the Mount,  Book II, Chapters 20