37 But let your speech be yea, yea: no, no: and that which is over and above these, is of evil.
Rabanus: He instructs us how we ought to speak. . . . To affirm any thing it is sufficient to say, It is so; to deny, to say, It is not so.
Or, “yea, yea; nay, nay,” are therefore twice repeated, that what you affirm with the mouth you should prove in deed, and what you deny in word, you should not establish by your conduct.
Hilary: They who live in the simplicity of the faith have not need to swear, with them ever, what is is, what is not is not; by this their life and their conversation are ever preserved in truth.
Augustine: He who has learned that an oath is to be reckoned not among things good, but among things necessary, will restrain himself as much as he may, not to use an oath without necessity, unless he sees men loth to believe what it is for their good they should believe, without the confirmation of an oath.
This then is good and to be desired, that our conversation be only, “yea, yea; nay, nay; for what is more than this cometh of evil.” That is, if you are compelled to swear, you know that it is by the necessity of their weakness to whom you would persuade any thing; which weakness is surely an evil. What is more than this is thus evil; not that you do evil in this just use of an oath to persuade another to something beneficial for him; but it is an evil in him whose weakness thus obliges you to use an oath.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. I (London: Rivington, 1842).