I love this teaching of Jesus, and I too think it is central to the lives of “kingdom people” though, as an American I wish we could find a better term than “kingdom” to refer to those striving to live God’s vision of a just and compassionate world. How about "the vision of God" rather than the "kingdom of God," and “vision people” rather than "kingdom people"?
And you are right to cite Leviticus 19:2 and Numbers 30:2. We should, given the nature of this dialogue, also mention the third commandment against taking God’s name in vain (Exodus 20:7) and the ninth commandment against swearing false witness against one’s neighbor (Exodus 20:16). It is clear, I think, that biblical Judaism has no problem with swearing oaths if done so with integrity. But by the time of Jesus an anti-oath movement was well under way.
Here is a passage from the Book of Enoch written by a contemporary of Philo some decades before Matthew: “I promise you, my children, that I will not swear by a single oath; neither by heaven nor by earth, nor by anything else made by God. God said, ‘There is no swearing in me, nor injustice, but truth.’ If there is no truth in men, let them swear by a word—Yea, yea, or Nay, nay.” (49: 1-2)
Enoch seems to understand “yea, yea” and “nay, nay” as oaths, as do the rabbis in the Talmud (Shavuot 36a). Jesus’ brother James may be trying to explain Jesus’ teaching in light of the rabbinic notion that “yea yea” and “nay nay” are also oaths when he says, “But above all my brethren, do not swear, either by heaven or by earth or with any oath. But let your “Yes” be “Yes,” and your “No” be “No” (James 5: 10-14). In this rendering of the teaching the double “yes” and “no” are not formulae as in the rabbis’ thinking but simply a way of saying, “Say what you mean, and do what you say.” If this is what Jesus meant then there is no conflict with the rabbis of his day.
Philo also argues against taking oaths, “That being who is the most beautiful, and the most beneficial to human life, and suitable to rational nature, swears not, because truth on every point is so innate within him that his bare word is accounted an oath,” (On the Decalogue 17, M. 2).
In Ecclesiasticus (the Wisdom of Ben Sirach) we find something similar, “Accustom not your mouth to swearing. Neither habituate yourself to using the name of the Holy One,” (23: 9-11). The Essenes, too, protested against the taking of oaths, claiming that their word was stronger than any oath, and arguing that swearing an oath was worse than perjury (Josephus, Jewish Wars, 2. 8, 6-7).
Given all of this we can see that Jesus’ position was not unique to him, and that he is simply taking sides in an ongoing Jewish debate on swearing oaths. But why this is so important? It could be that people were using the oaths and invoking the Name of God in support of lies, and Philo, Sirach, the Essenes, Jesus, and James are calling people to a higher level of integrity and honesty. This is important, but somewhat prosaic.
Looking for something more challenging, we should note that in Jesus’ day swearing an oath of fidelity to Caesar was a major concern. Josephus in his Antiquities (15:368, and 17:42) tells of two cases when the Jews refused to take oaths to Rome. The first was a combined effort of both Pharisaic schools and the Essenes who refused to swear a loyalty oath to Herod. The second was a general Pharisaic refusal to swear an oath of loyalty to Caesar. Rome responded with uncharacteristic restraint, fining the offenders rather than killing them.
Maybe Jesus is using the oath issue with Rome in mind, making his teaching yet another act of nonviolent resistance to Roman occupation. By refusing to take oaths one essentially denies the absolutist claims of the object of the oath, in this case Caesar. When Quakers refuse to pledge allegiance to the flag of the United States they are resisting the temptation of many patriots to equate God and country. Since to most Romans Caesar was God, not taking oaths was a way of affirming political atheism.
The question for me is, How does this translate into living the “vision of God” today? Certainly being honest and avoiding spin is part of it, but there must be something more. I wonder if our money, with the phrase “In God We Trust” printed on it is a kind of idol, and that participating in the American economic system is a betrayal of the vision of God? I really don’t know, but I would love to hear from you on this.