The Law permitted swearing oaths. In Matthew 5:33, Jesus quotes the first part of Leviticus 19:12 along with Numbers 30:2 and (possibly) Deuteronomy 23:21. Oaths were used in both legal and religious contexts. A promise between two people might include oaths. One vivid example is David swearing an oath to Bathsheba that her son would be the next king (1 Kings 1:29-30).
A vow often follows the form of “if, then.” A person might make a vow to God asking him for something. If God acts, the worshiper would then fulfill their vow. In 1 Samuel 1:1-11 Hannah makes a vow to God: If God gives her a child, she will dedicate that child to the Lord as a Nazarite.
The Law recognized the possibility of a rash vow. In Leviticus 5:4-6 the one who has made a rash vow can confess their sin and makes an appropriate sacrifice. The judge Jephthah is well-known for making a rash vow (Judg 11:30-31).
In the Mishnah, there is an entire section on vows, Nedarim. (The Hebrew word נֶדֶר, neder means “vow.”)
Nedarim 3:1 “Four [types of] vows did sages declare not binding: (1) Vows of incitement, (2) vows of exaggeration, (3) vows made in error, and (4) vows [broken] under constraint.”
This section of the Mishnah includes a series clarifications on how vows are interpreted, such as “He who vows not to drink wine is permitted to eat a cooked dish which has the taste of wine” (6:7) and “He who takes a vow not to have wine is permitted to have apple wine” (6:9). There is a discussion of loosing a vow in particular circumstances, such as “They unloose [vows] by reference to festival days and Sabbaths. At first they said, “On those particular days [the vows] are not binding, but for all other days they are binding” (9:6). A father may loose the vow of his betrothed daughter (10:1, the rest of the section discusses how that passed to various people if the father dies, lest the poor girl keep her own vows!)
There is a remarkable parallel in 2 Enoch 49:1-3. The writer of this paragraph emphasizes speaking the truth as opposed to swearing an oath. A potential problem with 2 Enoch is the possibility the text has been influence by the words of Jesus in the transmission process.
2 Enoch 49:1-2“For I am swearing to you, my children—But look! I am not swearing by any oath at all, neither by heaven nor by earth nor by any other creature which the Lord created. For ‹|the Lord|› said, ‘There is no oath in me, nor any unrighteousness, but only truth.’ So, if there is no truth in human beings, then let them make an oath by means of the words ‘Yes, Yes!’ or, if it should be the other way around, ‘No, No!’
The wisdom literature has much to say about keeping one’s word. For example, Ecclesiastes 5:8, “When you vow a vow to God, do not delay paying it, for he has no pleasure in fools.” Sirach 41:19 considers breaking an oath as shameful as bad manners: “Be ashamed of breaking an oath or agreement, and of leaning on your elbow at meals” (NRSV). Likewise, Sirach 18:22-23 says:
Sirach 18:22–23 (NRSV) Let nothing hinder you from paying a vow promptly, and do not wait until death to be released from it. Before making a vow, prepare yourself; do not be like one who puts the Lord to the test.
The Qumran Community reach a similar conclusion to Jesus. Lacy K. Crocker, points out “The Temple Scroll, however, based on an interpretation of Deuteronomy 23:21–23, emphasizes that it is better to abstain from making a vowing in order to avoid committing a transgression by failing to fulfill one’s vow” (see Josephus, JW 2.135 for the rejection of oaths by the Essenes.)
The point here is that there was an ongoing discussion at the time of Jesus over what constituted a binding oath and how one might get out of a vow if necessary. Some writers thought an oath could be made in such a way as to allow for a way out. Others warn against this sort of maneuvering as coming too close to breaking an oath to risk the wrath of God. Better to avoid making oaths at all.
But at the core of keeping one’s oaths is simple honesty. If someone does not keep their promises, they are dishonest. That Jesus would demand his disciples speak the truth is no surprise, he is standing on the Hebrew Bible. In Zechariah 8:17, the Lord himself declares “love no false oath, for all these things I hate.”
Before looking at the details of Jesus’s words on oaths, it is worth pausing and asking what it means to me “people of the truth.” Is telling the truth something which is non-negotiable for the disciple of Jesus? What about a foolish oath? Or a promise made without all of the information? What about saying something to win an argument which is not entirely true (but not totally false either). Can the true disciple of Jesus tolerate deception even if the results are positive? Think about the average Facebook post in these politically troubled times. Can the disciple of Jesus really resort to “alternative facts”?