Matthew 5:36

"Neither by thy head," saith He, "because thou canst not make one hair white or black." Here again, not as wondering at man, hath He withdrawn him from swearing by his head (for so man himself would be worshipped), but as referring the glory to God, and signifying that thou art not master even of thyself, and of course therefore not of the oaths made by thy head. For if no one would give up his own child to another, much more will not God give up His own work to thee. For though it be thy head, yet is it the property of another; and so far from being master thereof, thou shalt not be able to do with it, no not the least thing of all. For He said not, "Thou canst not make one hair grow;" but, "Not so much as change its quality."

Lastly, consider that the Saviour does not here forbid to swear by God, but by the Heaven, the Earth, by Jerusalem, by a man's head. For this evil practice of swearing by the elements the Jews had always, and are thereof often accused in the prophetic writing's. For he who swears, shows either reverence or love for that by which he swears. Thus when the Jews swore by the Angels, by the city of Jerusalem, by the temple and the elements, they paid to the creature the honour and worship belonging to God; for it is commanded in the Law that we should not swear but by the Lord our God.