The first holy oath?
Within Judaism, the practice of swearing an oath upon something holy is said to go back to one that Abraham required his servant to swear. As God was called upon to witness the proceedings, its setting would have been a peace offering, and as it involved the future of Abraham’s priestly cast, Abraham would logically have officiated. The servant, placing his hand under Abraham’s thigh, recited the oath (Gen 24:2-3). Why the thigh? As the Law of Moses later stated that the right thigh of any peace offering was to be set apart as the portion for the priest who officiated, i.e. it was treated as holy (Lev 7:33, Num 6:20). The same practice appears to provide the context for the servant’s oath. In requesting this oath Abraham was, in effect, entrusting the future of his priest’s portion into the hand of his servant, hence the symbolic gesture.
(Matt 5:33-37 WEB)5:33 “Again you have heard that it was said to them of old time, ‘You shall not make false vows, but shall perform to the Lord your vows,’
5:34 but I tell you, don’t swear at all: neither by heaven, for it is the throne of God;
5:35 nor by the earth, for it is the footstool of his feet; nor by Jerusalem, for it is the city of the great King.
5:36 Neither shall you swear by your head, for you can’t make one hair white or black.
5:37 But let your ‘Yes’ be ‘Yes’ and your ‘No’ be ‘No.’ Whatever is more than these is of the evil one.”
Stealing with words
Jesus had just taught on the fifth commandment and then the sixth commandment. As a Rabbi would often work through a passage of scripture, Jesus’ audience would therefore naturally be expecting him to continue with the seventh commandment, “You shall not steal” (Exod 20:15 WEB). What they heard was Jesus quoting a different verse. Yet he was still teaching about theft. For oaths were given to secure the reliability of a person’s words and, in an economy where bartering was commonplace and such agreements were essential to commerce, a transaction bound by an oath should have been sacrosanct.
Your agreements give something to others and represent a very real exchange. So, failing to keep your word steals something from the person to whom you have given it. When your ‘yes’ becomes ‘no’ or your ‘no’ becomes ‘yes’ then you steal at least another's trust in you, if not something greater. When you guaranteed your word by swearing on something for which God has responsibility, then, should you break your word, you steal God’s reputation. God’s decisions are unchanging and, if we follow his example, then ‘yes’ or ‘no’ are enough. Only if your intent is evil will you need to support your words with oaths. On these grounds, many Christians have refused to swear official oaths, often at considerable cost to themselves.
. . . commentary continues with Matt 5:38-42