Jesus then gives the case of oaths:
‘Again, you have heard that it was said to the people long ago, “Do not break your oath, but fulfil to the Lord the oaths you have made.” But I tell you, do not swear an oath at all: either by heaven, for it is God’s throne; or by the earth, for it is his footstool; or by Jerusalem, for it is the city of the Great King. And do not swear by your head, for you cannot make even one hair white or black. All you need to say is simply “Yes,” or “No”; anything beyond this comes from the evil one. (Mat 5:33-37)
To understand this teaching it is necessary to clearly understand the law of Moses on oaths. First, the teaching on witnesses in legal proceedings:
If a malicious witness takes the stand to accuse someone of a crime, the two people involved in the dispute must stand in the presence of the Lord before the priests and the judges who are in office at the time. The judges must make a thorough investigation, and if the witness proves to be a liar, giving false testimony against a fellow Israelite, then do to the false witness as that witness intended to do to the other party. You must purge the evil from among you. The rest of the people will hear of this and be afraid, and never again will such an evil thing be done among you. Show no pity: life for life, eye for eye, tooth for tooth, hand for hand, foot for foot. (Deut 19:16-21)
When witnesses give testimony in court there are a number of aspects that are to be assumed inherent in the process:
1. Witnesses are always put under oath. Unsworn testimony is inadmissible.
2. The court is in the business of hearing evidence for the purpose of imposing a forceful remedy on the one found to be the wrongdoer, as the means either to make him pay compensation to the victim or to punish the wrongdoer.
3. The witnesses are always put under oath to have them acknowledge that their testimony will be used as in point 2. and that they understand that the court will impose the same treatment on them should they be caught lying under oath.
The reason for the fear is also obvious: the court instils fear by threatening and carrying out violence against those who it judges.
This function of oaths can be seen in the places where the law of Moses requires that oaths be taken to settle disputes judicially, for example:
‘If anyone gives a donkey, an ox, a sheep or any other animal to their neighbour for safekeeping and it dies or is injured or is taken away while no one is looking, the issue between them will be settled by the taking of an oath before the Lord that the neighbour did not lay hands on the other person’s property. The owner is to accept this, and no restitution is required. (Ex 22:10-11)
In this type of case the function of the oath is to back up the truth of the claim not to have stolen the goods by calling upon oneself of the violent curses of the law should one later be proven to have lied. This is the nature of all oaths made by witnesses before the court, whether by the plaintiff, the respondent or third-party witnesses.
Oaths are also used in general commerce to back the truth of assertions or the faithful performance of obligations by advance appeal to the forceful remedies of the law. The commercial oath is, in effect, calling down on oneself the violent curses of the law as the basis upon which one will tell the truth or perform obligations. This is what it meant for an oath to be binding – the legal argument at the time of Jesus was about which oaths were binding (and would therefore make valid appeal to the force of the law):
‘Woe to you, blind guides! You say, “If anyone swears by the temple, it means nothing; but anyone who swears by the gold of the temple is bound by that oath.” You blind fools! Which is greater: the gold, or the temple that makes the gold sacred? You also say, “If anyone swears by the altar, it means nothing; but anyone who swears by the gift on the altar is bound by that oath.” You blind men! Which is greater: the gift, or the altar that makes the gift sacred? Therefore, anyone who swears by the altar swears by it and by everything on it. And anyone who swears by the temple swears by it and by the one who dwells in it. And anyone who swears by heaven swears by God’s throne and by the one who sits on it. (Mat 23:16-22)
In addition, in other non-commercial circumstances, people can also use oaths to invoke the violence of the court as the basis on which they will tell the truth (or to lie more convincingly):
Then he began to call down curses, and he swore to them, ‘I don’t know the man!’ (Mat 26:74)
However, oaths can be used to formalise matters that are not subject to litigation or court-ordered remedies, such as God’s covenants with man, man’s spiritual commitments to God, and man’s commitments to himself. They can also be used for rhetorical effect.
In light of this background on oaths, the teaching of Jesus, and its motivation can be understood: it is the ‘evil’ (Mat 5:37) of appeal to violent legal consequences (implied in oaths) that Jesus is prohibiting, and the purpose of the teaching is so that we will not ‘be condemned’ (James 5:12) by the court to suffer its violence (compare James 2:6). He is requiring men to do business without recourse to violent legal consequences. And he is prohibiting men from seeking or enabling such violent legal consequences by giving evidence to the court under oath.
It also explains why Jesus is not necessarily prohibiting oaths concerning purely spiritual matters, or oaths for rhetorical effect (as used by Paul in his letters).
Jesus demonstrates the violence of the court resulting from testifying under oath during his trial:
The high priest said to him, ‘I charge you under oath by the living God: Tell us if you are the Messiah, the Son of God.’
‘You have said so,’ Jesus replied. ‘But I say to all of you: from now on you will see the Son of Man sitting at the right hand of the Mighty One and coming on the clouds of heaven.’
Then the high priest tore his clothes and said, ‘He has spoken blasphemy! Why do we need any more witnesses? Look, now you have heard the blasphemy. What do you think?’
‘He is worthy of death,’ they answered. (Mat 26:63-66)
In this case Jesus, the makes his final and formal appeal to his people. It was this that Judas Iscariot had gambled on bringing Jesus into his rightful leadership position of the people of Israel against the Romans. However, in some sense the court oath of Jesus was a special case connected with his sacrificial death and in this sense not an example for us of how to avoid enabling the violence of the court generally.