The commandment against murder is now brought to its root. For the Law restrained the hand, but the Lord heals the heart. One may abstain from the outward deed and yet be guilty inwardly, if anger is harbored within. Thus He condemns not only the shedding of blood, but that passion from which the deed itself springs.

There are degrees in this inward disorder, and they are shown by the ways anger manifests itself. Anger may remain hidden in the heart; it may break forth in speech that wounds; or it may harden into settled contempt, by which one person is despised as though worthless. These are not light matters, but successive steps away from charity, each more grievous than the last, because each more completely destroys the bond of love between brethren.

The language of judgment, council, and fire does not describe earthly courts, but signifies the increasing seriousness of the fault. For as the inward sin deepens, so too does the guilt before God. The Lord thus teaches that what men excuse as small movements of the soul are, in truth, already subject to divine judgment.

Therefore reconciliation is placed before sacrifice. God does not delight in offerings brought by hands that conceal hostility in the heart. Even if one has not injured another outwardly, the remembrance that a brother is offended is enough to require healing first. Peace with man must precede worship of God, because love of neighbor is inseparable from love of Him who commands it.

The warning about the adversary urges haste in repentance. While one is still on the way—that is, while life endures—there is time to be reconciled, to amend what has been done, and to quiet the accusing voice of conscience. But once judgment has come, nothing remains to be changed. Thus the Lord calls not merely for restraint of violence, but for the rooting out of anger itself, so that righteousness may exceed that which is merely external and may begin within the soul.

Source: St. Augustine, On the Lord’s Sermon on the Mount,  Book I, Chapters 9–10