“Neither shalt thou swear by thy head, because thou canst not make one hair white or black.”
Matthew 5:36 — Greek Interlinear
Church Fathers on Matthew 5:36
The Lord has hitherto taught to abstain from injuring our neighbour, forbidding anger with murder, lust with adultery, and the putting away a wife with a bill of divorce. He now proceeds to teach to abstain from injury to God, forbidding not only perjury as an evil in itself, but even all oaths as the cause of evil, saying, 'Ye have heard it said by them of old, Thou shalt not forswear thyself.'
This was allowed under the Law, as to children; as they offered sacrifice to God, that they might not do it to idols, so they were permitted to swear by God; not that the thing was right, but that it were better done to God than to daemons.
For no man can swear often, but he must sometimes forswear himself; as he who has a custom of much speaking will sometimes speak foolishly.