Then it shall be, that whatsoever cometh forth of the doors of my house to meet me, when I return in peace from the children of Ammon, shall surely be the LORD's, and I will offer it up for a burnt offering.

King James Translators' Notes

whatsoever...: Heb. that which cometh forth, which shall come forth

and...: or, or I will offer it, etc

Jamieson-Fausset-Brown Bible Commentary

31. whatsoever cometh forth of the doors of my house to meet me-This evidently points not to an animal, for that might have been a dog; which, being unclean, was unfit to be offered; but to a person, and it looks extremely as if he, from the first, contemplated a human sacrifice. Bred up as he had been, beyond the Jordan, where the Israelitish tribes, far from the tabernacle, were looser in their religious sentiments, and living latterly on the borders of a heathen country where such sacrifices were common, it is not improbable that he may have been so ignorant as to imagine that a similar immolation would be acceptable to God. His mind, engrossed with the prospect of a contest, on the issue of which the fate of his country depended, might, through the influence of superstition, consider the dedication of the object dearest to him the most likely to ensure success.

shall surely be the Lord's; and [or] I will offer it up for a burnt offering-The adoption of the latter particle, which many interpreters suggest, introduces the important alternative, that if it were a person, the dedication would be made to the service of the sanctuary; if a proper animal or thing, it would be offered on the altar.

Matthew Henry's Concise Commentary

11:29-40 Several important lessons are to be learned from Jephthah's vow. 1. There may be remainders of distrust and doubting, even in the hearts of true and great believers. 2. Our vows to God should not be as a purchase of the favour we desire, but to express gratitude to him. 3. We need to be very well-advised in making vows, lest we entangle ourselves. 4. What we have solemnly vowed to God, we must perform, if it be possible and lawful, though it be difficult and grievous to us. 5. It well becomes children, obediently and cheerfully to submit to their parents in the Lord. It is hard to say what Jephthah did in performance of his vow; but it is thought that he did not offer his daughter as a burnt-offering. Such a sacrifice would have been an abomination to the Lord; it is supposed she was obliged to remain unmarried, and apart from her family. Concerning this and some other such passages in the sacred history, about which learned men are divided and in doubt, we need not perplex ourselves; what is necessary to our salvation, thanks be to God, is plain enough. If the reader recollects the promise of Christ concerning the teaching of the Holy Spirit, and places himself under this heavenly Teacher, the Holy Ghost will guide to all truth in every passage, so far as it is needful to be understood.