52. But it may be asked why, when it was said, “But I say unto you, Swear not at all,” it was added, “neither by heaven, for it is God’s throne,” etc., up to “neither by thy head.” I suppose it was for this reason, that the Jews did not think they were bound by the oath, if they had sworn by such things: and since they had heard it said, “Thou shalt perform unto the Lord thine oath,” they did not think an oath brought them under obligation to the Lord, if they swore by heaven, or earth, or by Jerusalem, or by their head; and this happened not from the fault of Him who gave the command, but because they did not rightly understand it. Hence the Lord teaches that there is nothing so worthless among the creatures of God, as that any one should think that he may swear falsely by it; since created things, from the highest down to the lowest, beginning with the throne of God and going down to a white or black hair, are ruled by divine providence. “Neither by heaven,” says He, “for it is God’s throne; nor by the earth, for it is His footstool:” i.e. , when you swear by heaven or the earth, do not imagine that your oath does not bring you under obligation to the Lord; for you are convicted of swearing by Him who has heaven for His throne, and the earth for His footstool.
Augustine of Hippo, Our Lord’s Sermon on the Mount 2.17