Was Jesus setting Numbers 30:2 aside to promote Matthew 5:34?
This question relates directly to the distinction between the Law of Moses (which has been "set aside") and the Law of Christ. We can know this when we consider the Letter to the Hebrews:
Hebrews 7:11-12: "Now if perfection was through the Levitical priesthood (for on the basis of it the people received the Law), what further need was there for another priest to arise according to the order of Melchizedek, and not be designated according to the order of Aaron? 12For when the priesthood is changed, of necessity there takes place a change of law also."
Here is another instance where we can know that many of the laws of the O/T, such as making vows, have been done away (or revised) in Christ:
Galatians 3:23-24a: "But before faith came, we were kept in custody under the law [of Moses], being shut up to the faith which was later to be revealed [in Christ]. Therefore the Law [of Moses] has become our tutor to lead us to Christ..."
These passages are emblematic of the distinction between the Law of Moses (the Old Covenant) and the Law of Christ (the New Covenant) — Christ being our High Priest. Not only was the priesthood changed but so too was the entire Mosaic system upon which all else rested. Some may disagree with this observation, instead choosing to believe that the change was minor.
Nonetheless, the Mosaic order was reinstituted in another form upon the inauguration of the new priesthood, one in which all Christians (saints) partake as priests (cf. 1 Pet. 2:9, and a temple: 1 Cor. 6:19).
As pointed out in the OP, this is particularly significant based on Jesus' Sermon on the Mount, in which He repeatedly enumerated many of the O/T laws beginning with "You have heard it said..." while replacing them with His new, more stringent commands (paraphrased):
"You have heard it said":
- "You shall not commit murder, but he who hates his brother has committed murdered in his heart" (Matt. 5:21-22).
- "You shall not commit adultery, but he who lusts in his heart has committed adultery..." (Matt. 5:27-28).
- "He who divorces his wife and marries another commits adultery" (Matt. 5:31-32).
- "You shall make no false vows... But I say make no oath at all" (Matt. 5:33-37).
- "An eye for an eye... turn the other cheek" (Matt. 5:38-39).
- "You shall love your neighbor and hate your enemy... love your enemies, pray for those who persecute you" (Matt. 5:43-44).
With regard to Matthew 5:19 from the OP, Coffman had the following to say:
In this verse, Christ plainly refers to his own commandments with the strong warning that men are under obligations to heed and observe the laws he gives. Today, there are some who speak of certain Scriptures as "mere command". But Christ made his commandments to be of overwhelming importance and set forth the principle that "the least" of his commandments was to be received and honored with [respect] and obedience (emphasis added).
The 613 laws and ordinances under the Mosaic regime have been superseded by a stricter set of laws that comprise the Law of Christ (Gal. 6:2), or the Law of Liberty (Jas. 1:25, 2:12), or the Royal Law (Jas. 2:8): there are other equivalent designations of the perfect Law of Christ (Jas. 1:25).
Christ nailed the Law of Moses to the Cross (Col. 2:14), by introducing His Own commandments (only a few of which are listed above). The new laws are far more comprehensive than anything written in the Old Testament because they apply to our thoughts as well as our actions.
A passage in the Letter of James appears to settle the matter regarding vows:
James 5:12: "But above all, my brethren, do not swear, either by heaven or by earth or with any other oath; but your yes is to be yes, and your no, no, so that you may not fall under judgment." (cf. Matt. 5:37).
Yes, Christ has "set aside" the Law of Moses where there has indeed been a change in the Law — and that includes making vows.