23 Thus, as regards murder, which was understood to mean merely the destruction of the body, by which a man is deprived of life, the Lord explained that every unjust disposition to injure our brother is a kind of murder. So John also says, "He that hates his brother is a murderer." And as it was thought that adultery meant only the act of unlawful intercourse with a woman, the Master showed that the lust He describes is also adultery. Again, because perjury is a heinous sin, while there is no sin either in not swearing at all or in swearing truly, the Lord wished to secure us from departing from the truth by not swearing at all, rather than that we should be in danger of perjury by being in the habit of swearing truly.
For one who never swears is less in danger of swearing falsely than one who is in the habit of swearing truly. So, in the discourses of the apostle which are recorded, he never used an oath, lest he should ever fall unawares into perjury from being in the habit of swearing. In his writings, on the other hand, where he had more leisure and opportunity for caution, we find him using oaths in several places, to teach us that there is no sin in swearing truly, but that, on account of the infirmity of human nature, we are best preserved from perjury by not swearing at all. These considerations will also make it evident that the things which Faustus supposes to be peculiar to Moses were not destroyed by Christ, as he says they were.
Source: Reply to Faustus the Manichaean (New Advent)