This law was delivered to the heads of the tribes that they might instruct those who were under their charge, explain the law to them, give then necessary cautions, and call them to account, if there were occasion, for the breach of their vows. Perhaps the heads of the tribes had, upon some emergency of this kind, consulted Moses, and desired by him to know the mind of God, and here they are told it: This is the thing which the Lord has commanded concerning vows, and it is a command still in force.
1. The case supposed is that a person vows a vow unto the Lord, making God a party to the promise, and designing his honour and glory in it. The matter of the vow is supposed to be something lawful: no man can be by his own promise bound to do that which he is already by the divine precept prohibited from doing. Yet it is supposed to be something which, in such and such measures and degrees, was not a necessary duty antecedent to the vow. A person might vow to bring such and such sacrifices at certain times, to give such and such a sum or such a proportion in alms, to forbear such meats and drinks which the law allowed, to fast and afflict the soul (which is specified Num 30:13) at other times besides the day of atonement. And many similar vows might be made in an extraordinary heat of holy zeal, in humiliation for some sin committed or for the prevention of sin, in the pursuit of some mercy desired or in gratitude for some mercy received. It is of great use to make such vows as these, provided they be made in sincerity with due caution. Vows (say the Jewish doctors) are the hedge of separation, that is, a fence to religion. He that vows is here said to bind his soul with a bond. It is a vow to God, who is a spirit, and to him the soul, with all its powers, must be bound. A promise to man is a bond upon the estate, but a promise to God is a bond upon the soul. Our sacramental vows, by which we are bound to no more than what was before our duty, and which neither father nor husband can disannul, are bonds upon the soul, and by them we must feel ourselves bound out from all sin and bound up to the whole will of God. Our occasional vows concerning that which before was in our own power (Act 5:4), when they are made, are bonds upon the soul likewise. 2. The command given is that these vows be conscientiously performed: He shall not break his word, though afterwards he may change his mind, but he shall do according to what he has said. Margin, He shall not profane his word. Vowing is an ordinance of God; if we vow in hypocrisy we profane that ordinance: it is plainly determined, Better not vow than vow and not pay, Ecc 5:5. Be not deceived, God is not mocked. His promises to us are yea and amen, let not ours to him be yea and nay.

And Moses spake unto the heads of the tribes,.... Or the princes of them, who could more easily be convened, and who used to meet on certain occasions, and on whom it lay to see various laws put in execution:

concerning the children of Israel; how they ought to conduct and behave in the following case, it being an affair which concerned them all:

saying, this is the thing which the Lord hath commanded; relating to vows. Aben Ezra is of opinion that this was delivered after the battle with Midian, of which there is an account in the following chapter, and is occasioned by what was said, to the tribes of Gad and Reuben, Num 32:24.

do that which hath proceeded out of your mouth; to which they replied:

thy servants will do as my lord commandeth; upon which the nature of a vow, and the manner of keeping it, are observed; but the occasion of it rather seems to be what is said towards the close of the foregoing chapter, Num 29:39, that the various sacrifices there directed were to be offered in their season, besides the vows and freewill offerings; and when these were ratified and confirmed, and when null and void, and to be fulfilled or neglected, is the principal business of this chapter.