If a man, &c.— That is, a person entirely master of himself, and actually endowed with reason. See Leviticus 27. The Jewish rabbis have advanced two excellent maxims upon this subject: the one, that a good man, who performs his duty, does well not to lay himself under the obligation of vows; the other, that we are not permitted to make any other vows than those which in their own nature tend to the glory of God. It is very evident, that vows respecting things lawful only must be here meant; and, probably, Moses principally refers to such persons as, intending to glorify God, made vows to offer some sacrifice of supererogation upon the feast days, or to afflict their souls on some other day than that of expiation. No vows nor oaths can bind to that which is unlawful. See Grotius de Jure B. & P. lib. 2: cap. 13 sect. 6. The determination of Philo upon this head is very just; that he who perpetrates any act of injustice upon account of his oath, adds one crime to another; first, by taking an unlawful oath; and then, by doing an unlawful action; therefore, such a one ought to abstain from the unjust action, and pray God to pardon him for his rash oath; an observation which clearly shews the iniquity of Herod's conduct, Matthew 14:9. Religious vows were common among all nations, wherein mankind seem to have considered God rather after the manner of men, than suitably to his own nature. We may observe, however, that the vows which we most frequently read of in the Old Testament, were only solemn resolutions of universal obedience to God. See Genesis 28:20. 2 Samuel 15:8. Isaiah 19:21. Jonah 1:16; Jonah 2:9. The solemn vows and resolutions which Christians come under by baptism and the Lord's supper are of this kind: and such holy renewals, and confirmations of our resolutions to obey God, are undoubtedly of great and perpetual use. Other vows serve only for a snare, and it is most prudent to omit them.
Break his word— "Profane," says Dr. Beaumont, i.e. "not violate or break his promise. For the like phrases, see Psalms 55:20; Psalms 89:34. All that proceedeth, &c. signifies every word of his; as the phrase is explained, Luke 4:4 from Deuteronomy 8:3 and this of vows lawful, and in a man's power to perform."