Numbers 30:2 - Joseph Benson’s Commentary on the Old and New Testaments

If a man vow a vow Concerning something lawful, and in his power to perform. Unto the Lord To the honour and service of God. Or swear an oath Confirm his vow by an oath. To bind his soul with a bond To restrain himself from something otherwise lawful; as, suppose, from such a sort of meat or drink; or to oblige himself to the performance of something otherwise not necessary, as to observe a private day of fasting. He shall not break (Hebrew, he shall not profane) his word Not render his word, and consequently himself, profane, or contemptible in the eyes of others. He shall do according to all that proceedeth out of his mouth Punctually and conscientiously. His vow shall be performed in the manner, time, and kind which was at first proposed, in reverence to the great God to whom it was made. But in case a man vows, or takes an oath, to do any thing that is in itself unlawful, as those Jews did, mentioned Acts 23:14, nothing can be plainer than that such vow or oath must be void in the very nature of the thing. For promises and resolutions, enforced by the strongest oaths, or most solemn vows, are but secondary obligations, and therefore can never absolve us from our primary and immutable obligation to obey the laws of God and nature; for this would be to say, that we could, by an oath, oblige ourselves to do what God had before obliged us not to do. “He who perpetrates any act of injustice,” says Philo Judæus, de specialibus legibus, “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.” Thus Herod ought to have done; instead of performing the rash promise which he had sealed with an oath, he ought to have punished that wicked woman, who instigated him to commit murder, under pretence of fulfilling his oath, Matthew 14:9. Grotius observes further, that though the thing promised be not absolutely unlawful, yet, if it obstruct some greater moral good, such a promise, even sealed with an oath, is not binding.