The Lord now turns to the purification of speech, teaching that truthfulness should so govern the soul that oaths become unnecessary. For the Law restrained false swearing, but Christ calls His disciples to such integrity of heart that they do not need to bind their words by adjurations. The abundance of oaths does not arise from reverence, but from mistrust; and where truth is firmly rooted, swearing is superfluous.

When men swear by heaven, or earth, or Jerusalem, they suppose they avoid calling God as witness; yet all these belong to Him and are sustained by His power. Even the head, by which one may think to swear lightly, is not one’s own possession, since no man can change even a single hair by his will. Thus every oath, however indirect, ultimately refers back to God, before whom all truth stands naked.

The Lord does not forbid all swearing without distinction, but He warns against the habit of swearing that springs from a divided or unreliable heart. For the disciple is not to use oaths to prop up uncertain speech, but to cultivate such fidelity that his simple word suffices. Where “Yes” truly means yes, and “No” truly means no, speech itself bears the mark of righteousness.

What exceeds this simplicity proceeds from evil, not because the created things by which men swear are evil, but because the necessity of such excess reveals a fault within. Either truth is not loved for its own sake, or fear of man intrudes where fear of God should reign. Thus the Lord leads His hearers from the outward regulation of words to the inward formation of honesty, so that the tongue may faithfully express a heart already ordered in truth.

Source: St. Augustine, On the Lord’s Sermon on the Mount,  Book I, Chapters 13-14