Again, you have heard that it has been said by them of old time, You shall not perjure yourself, but shall perform unto the Lord your oaths:
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Augustine of Hippo
AD 430
The righteousness of the Pharisees is not to forswear oneself; and this is confirmed by Him who gives the command not to swear, so far as relates to the righteousness of the kingdom of heaven. For just as he who does not speak at all cannot speak falsely, so he who does not swear at all cannot swear falsely. But yet, since he who takes God to witness swears, this section must be carefully considered, lest the apostle should seem to have acted contrary to the Lord's precept, who often swore in this way, when he says, Now the things which I write unto you, behold, before God I lie not; and again, The God and Father of our Lord Jesus Christ, which is blessed for evermore, knows that I lie not. Of like nature also is that asseveration, For God is my witness, whom I serve with my spirit in the gospel of His Son, that without ceasing I make mention of you always in my prayers. Unless, perchance, one were to say that it is to be reckoned swearing only when something is spoken of by which one ...
Again, ye have heard, &c. Thou shall perform, i.e, Thou shalt pay, shalt fulfil what thou hast sworn unto the Lord, or by the Lord that thou wilt do. So S. Chrysostom properly explains that by oaths are here meant vows confirmed by an oath, that we are bound to render them, that Isaiah , perform them unto God. Suarez explains differently. "If thou desirest to swear, swear by the true God, not by idols."
The law had prescribed a penalty for false swearing, so that the ritual of an oath might hold false testimony in check. … But faith removes the need for using an oath. It establishes in truth the dealings of our life. Once the inclination to deceive has been checked, it enjoins simplicity in speaking and hearing. … Therefore those who are living in the simplicity of faith have no need for the ritual of an oath. With such people, what is, always is, and what is not, is not. For this reason, their every word and deed are always truthful.
Why did He go straightway not to theft, but to false witness, passing over that commandment? Because he that steals, does upon occasion swear also; but he that knows not either swearing or speaking falsehood, much less will he choose to steal. So that by this He has overthrown the other sin likewise: since falsehood comes of stealing.
