Jonathan Edwards believes that Jephthah did not put his daughter to death but devoted her to perpetual virginity. At first I was skeptical, but his comparisons to Hebrews 11 and several of the women devoted to virginity in the NT as well as commands to Widows makes this seem even more likely. So after reading it, I think Edwards is right on it. I've quoted the pages in full below, but you may read it directly from Yale here:
Note 223. Judges 11:3040.
Concerning JEPHTHAH'S VOW and his offering up his daughter. That Jephthah did not put his daughter to death and burn her in sacrifice, the following things evince.
by Jonathan Edwards
The tenor of his vow, if we suppose it to be a lawful vow, did not oblige him to it. He promised that whatsoever come forth of the doors of his house to meet him, should surely be the Lord's, and he would offer it up for a burnt offering. He was obliged to no more by this vow than only to deal with whatsoever come forth of the doors of his house to meet him, as those things that were holy to the Lord, and by right burnt offerings to God were to be dealt with, by God's own law and the rules that he had given. Supposing it had been an ass, or some unclean beast had come forth to meet him, as Jephthah did not know but it would, his vow would not have obliged him to have offered it in sacrifice, or actually to have made a burnt offering of it. But he must have dealt with it, as the law of God directed to deal with an unclean beast that was holy to the Lord, and that otherwise must have been actually a burnt offering to the Lord, had it not been for that legal incapacity of the impurity of its nature. All living things that were consecrated were to be as it were burnt offerings to God, i.e. they were actually to be offered up a burnt sacrifice, if not of a nature that rendered it incapable of this, and then in that case something else was to be done that God would accept instead of offering it up a burnt sacrifice. The direction we have in Leviticus 27:1113, "And if it be any unclean beast, of which they do not offer a sacrifice unto the Lord, then he shall present the beast before the priest; and the priest shall value it, whether it be good or bad. As thou valuest it, who art the priest, so shall it be. But if he will at all redeem it, then he shall add a fifth part thereunto of thy estimation"; i.e. it should be valued by the priest, and the man should, after it was valued, determine whether he would redeem it, or no. And if not, he was to break his neck if an ass (Exodus 13:1213); or if other unclean beast, it must be sold according to the priest's estimation, Leviticus 27:27 (as is elsewhere directed to be done to unclean beasts that were holy to the Lord, Exodus 34:20 ). But if he would redeem it, if it were an ass, he was to redeem it with a lamb (Exodus 13:1213); if other unclean beast, he was to add the fifth part to the priest's estimation, that is, he was to give the value of the beast, and a fifth part more. And if Jephthah had done this in case an unclean beast had met him, he would have done according to his vow. If he had, in such a case, gone about to have offered an unclean beast as a burnt sacrifice, he would dreadfully have provoked God. His vow could be supposed to oblige him to no other than only to deal with the unclean beast that was consecrated, as the law of God directed to deal with it, instead of offering it a burnt offering. And so when it was his daughter that met him, he might do to her according to his vow, without making her a burnt sacrifice, if he did that to her which the law of God directed to be done to a dedicated person, instead of actually making them a burnt sacrifice, by reason of the incapacity which, by the mercy of God, attends a human person to be a burnt sacrifice. For to offer either a man or an unclean beast in sacrifice to God are both mentioned as a great abomination to God, and as what were universally known so to be. Isaiah 66:3, "He that killeth an ox is as if he slew a man; he that sacrificeth a lamb, as if he cut off a dog's neck; he that offereth an oblation, as if he offered swine's blood." But the more fully to clear up the difficulties that attend this matter, I will particular[ly] observe some things concerning the laws, that related to persons that were consecrated so as to become holy to the Lord.
First. Every living thing that was holy to the Lord, whether of man or beast, was by right a burnt offering to God, and must be either actually made a burnt sacrifice, or something else must be done to it, that God appointed to be in lieu of burning it in sacrifice. Thus the firstborn of man and beast, they were all holy to the Lord, and must either be offered up a burnt sacrifice, or to be redeemed. The firstborn of men and of unclean beasts were to be redeemed.
Second. Persons that were devoted to God by a singular vow (unless they were those that were devoted to be accursed, of which, Leviticus 27:2829), were to be brought and presented before the Lord, that the priest might estimate them; and they were to [be] redeemed according to the priest's estimation. But beasts that might be sacrificed were to be sanctified (Leviticus 27:79). See Synopsis Criticorum on Leviticus 27:2.
Third. Persons that were thus devoted to God by the vow of their parents were yet to remain persons separate, and set apart for God, after they were redeemed; and this may appear by several things.
1. The redemption was only to redeem them from being slain in sacrifice; it was not to redeem them from being holy to the Lord, or persons set apart and sanctified to him.
2. The firstborn were appointed to [be] given or consecrated to God (Exodus 13:2 and Exodus 23:19); and they were by God's law holy to the Lord, in the very same manner as persons devoted to him by a singular vow, as is evident because they were to be redeemed in the same manner and at the same price, as is evident by comparing the beginning of the Leviticus 27 with Numbers 18:1516. God, in giving the rule for the redemption of the firstborn in the latter place, evidently refers to what he had before appointed in the former place, concerning persons devoted by a singular vow. And so likewise, the firstlings of unclean beasts were to be redeemed, in the same manner as unclean beasts that were devoted, as appears by comparing Leviticus 27:1113 with Leviticus 27:27; but yet the firstborn still remained separated to God, as his special possession, after they were redeemed. Hence the Levites were accepted for the firstborn, to a tribe separated to God, after the firstborn were thus redeemed.
3. Persons that were devoted to God by the vow of their parents were Nazarites, as well as those that were separated by their own vows. The word "Nazarite"1 signifies "one that is separated"; they might be separated by their parents' vows, or their own. This is very evident in instances that we have in Scripture. Thus Samuel was a Nazarite by the vow of his mother. 1 Samuel 1:11, "And she vowed a vow, and said, O Lord of hosts, if thou wilt indeed look on the affliction of thine handmaid, and remember me, and not forget thine handmaid, but will give unto thine handmaid a manchild, then I will give [him] unto the Lord all the days of his life, and there shall no razor come upon his head." And so it was with respect to Sampson (Judges 13:5). But the Nazarite was to continue separated to God, as long as he remained under the vow by which he was devoted.
4. Those that were thus devoted to God to be Nazarites were, to the utmost of their power, to abstain from all legal pollutions (Lamentations 4:7). With respect to defilements by dead bodies, they were required to keep themselves pure, with greater strictness than the very priests, except the high priest alone, and were obliged to as [great] strictness as the high priest himself (Numbers 6:67, compared with Leviticus 21:1011). And though only some legal impurities are expressly mentioned, as what the Nazarite was to avoid, yet it is to be understood, that he is to his utmost to separate himself from all legal defilements, agreeable to his name, a Nazarite, or a "separate person." The Nazarite was to abstain from all legal impurities, in like manner as the priests, and even as the high priest; there are like directions given to one as to the other. The high priest was on no account to defile himself with the dead, and were forbidden to drink wine, or strong drink, when they went into the tabernacle of the congregation (Leviticus 10:9). The priests were to abstain from all manner of legal defilement, as far as in them lay (Leviticus 22:19).
If it be objected against this, that the Levites, who were accepted to be the Lord's instead of the firstborn that were holy to the Lord, were not obliged to such strictness, I answer, that this may be one reason why God did not look on the firstborn as being fully redeemed by the Levites' being substituted in their stead, but there were still extraordinary charges required of them for the maintenance of the Levites, much more than in proportion to the bigness of the tribe; and God might accept this as an equivalent for their not being so strictly separated, as he accepted extraordinary redemption money for the odd number of the firstborn that were more than the Levites (Numbers 3:4647, and Numbers 18:1516).
5. Those that were devoted to God to be Nazarites by a singular vow were to devote themselves wholly to religious exercises, and to spend their lives in the more immediate service of God. For though this ben't particularly expressed, but only some things are expressed that they should abstain from, yet this is implied in their being God's, his being separated to the Lord (Numbers 6:5), his being holy to the Lord. Numbers 6:6, "All the days that he separateth himself unto the Lord," he shall be holy; and Numbers 6:8, "All the days of his separation, he shall be holy unto the Lord." In like manner, as in the Second Commandment, there are only some things particularly mentioned that we should abstain from on the sabbath, but 'tis not expressly said that the day should be spent in religious exercises, yet 'tis implied in that, that the seventh day is the sabbath of the Lord our God, and that we are commanded to keep it holy. This was evidently Hannah's intention in her vow, whereby she devoted Samuel to be a Nazarite, as was explained by her own words and practice. 1 Samuel 1:28, "Therefore also I have lent him to the Lord; as long as he liveth he shall be lent to the Lord." And accordingly she brought him, and left him in the sanctuary to dwell continually there, and there to spend his time in sacred business. 1 Samuel 2:11, "And Elkanah went to Ramah to his house. And the child did minister unto the Lord before Eli the priest." 1 Samuel 2:18, "But Samuel ministered before the Lord, being a child, girded with a linen ephod."
6. It was necessary that a woman that was devoted to be a Nazarite (for a women might be a Nazarite, Numbers 6:2), that she should thenceforward avoid marrying, and refrain from all carnal intercourse with men. If she was a virgin when she was devoted, it was necessary that she should continue a virgin till her vow was ended; and if she was devoted for her whole life, she must continue a virgin forever. And if she was a widow, she must continue in her widowhood, and that on two accounts.
(1) Marrying would be contrary to the obligation, that has been taken notice of, that the Nazarite was under, with the utmost strictness avoid all legal defilements, for marrying unavoidably exposed to great legal impurities, and of long continuance. See Leviticus 12. There were scarcely any legal impurities to which the children of Israel were exposed, excepting the leprosy, that were so great as those that marriage brought women into. Being therefore devoted to God, to be holy to the Lord in the utmost possible legal purity, she must avoid marrying. And then those legal impurities rendered her incapable of those sacred offices and services that she was devoted to. It incapacitated her from conversing in holy things, or drawing near to God in ordinances, as much as being defiled by the dead body of a man incapacitated a priest from his work and office. Leviticus 12:4, "And she shall then continue in the blood of her purifying three and thirty days; she shall touch no hallowed thing, nor come into the sanctuary till the days of her purifying be fulfilled," which, in all, for a son made up forty days, and for a daughter fourscore days, which must needs be very inconsistent with the circumstances of the Nazarite, that was devoted wholly to attend on God and holy exercises in the way of the Jewish ordinances. If the Nazarite were a male, his marrying did not expose him to such legal impurities. The Nazarite was to observe as strict a legal purity as the high priest himself, as has been observed; but he, for the greater purity, was allowed to marry none but a virgin. Therefore doubtless the woman herself, that was a Nazarite, was obliged to continue a virgin. See how some things were required in the law of Moses by consequence, though not expressly, in my papers of "Infant Baptism."
(2) Marrying would utterly destroy the main design of her being dedicated in the vow of a Nazarite, which was, that she might be wholly devoted to the more immediate service of God in sacred things. If she was married, her time must unavoidably be exceedingly taken up in secular business and cares, in tending and bringing up children, and in providing for and taking care of a family, which exceedingly fills married women's hands and hearts, and is as inconsistent as possible with the design of the vow of the Nazarite. Hence the women that were devoted to the special service of God's house in the primitive church (though not devoted to God so solemnly, nor in so great a degree, as the Nazarite), must be one that was not married, and never like to marry; and it was looked upon and spoken of by the apostles as sinful in such to marry. 1 Timothy 5:11, "But the younger widows refuse; for when they have begun to wax wanton against Christ, they will marry." And the reason that is given why they should be widows, that were like ever to continue so, and free from all worldly care, was that they might be the more entirely at liberty for religious duties. 1 Timothy 5:35, "Honor widows that are widows indeed. But if any widow have children or nephews, let them learn first to show piety at home, and to requite their parents, for that is good and acceptable before God. Now she that is a widow indeed, and desolate, trusteth in God, and continueth in supplications and prayers night and day." Those widows in the primitive church seem to be, in some degree, in imitation of the Nazarites in the Jewish church. Anna the prophetess was in all probability a Nazarite, or one that, after her husband's death, had devoted herself to the service of God by such a vow as that we have been speaking of, and therefore continued in widowhood to so great an age, because her vow obliged her to it. And therefore she, throwing by all worldly care, devoted herself wholly to the immediate service of God. Luke 2:3637, "And there was one Anna, a prophetess, the daughter of Phanuel, of the tribe of Aser. She was of a great age, and had lived with an husband seven years from her virginity; and she was a widow of about fourscore and four years, which departed not from the temple, but served God with fastings and prayers, night and day," the like expression with that that the Apostle uses concerning widows (1 Timothy 5:5).
And therefore,9 when we have an account that after Jephthah's daughter had been let alone two months, to go up and down the mountains with her companions to bewail her virginity, that she returned to her father, who did to her according to his vow. That which Jephthah did was this: he took her up to the sanctuary before the Lord, and presented her before the priest, that he might estimate her, and then paid according to his estimation (Thus the Jews that came out of the captivity vowed that they would offer the firstborn of their sons, Nehemiah 10:36.), whereby she was redeemed from being made a burnt sacrifice, according to the law. And by thus presenting her in the sanctuary, and offering up that which [was] accepted instead of her blood, she was actually separated according to the vow. Her separation began from that time, and thenceforward she was to begin her strict abstinence from all legal impurities, and to spend her time in sacred offices. And 'tis probable that Jephthah thenceforward left her in the sanctuary, to dwell there as long as she lived, as Hannah did to her son Samuel, whom she had devoted to be a Nazarite. 1 Samuel 1:22, "I will not go up till the child be weaned, and then I will bring him, that he may appear before the Lord, and there abide forever"; and as the other Hannah, or Anna, did with herself after she had devoted herself to perpetual widowhood as a Nazarite, of whom we read, Luke 2:37, that she was widow of fourscore years old, and "departed not from the temple." And there probably Jephthah's daughter continued in supplications and prayers night and day, for she was eminently disposed and prepared for such duties by that remarkable spirit of piety that appeared in her, by her resignation with respect to the vow her father had made concerning her. And what time she did not spend in duties of immediate devotion, she might spend in making of priests' garments (Exodus 35:2526), or in other business subservient to the work of the sanctuary, as there might be enough found that a woman might do.
II. The nature of the case will not allow us to suppose that that was done, that was so horrid and so contrary to the mind and will of God, as putting of her to death, and offering her up as a burnt sacrifice. God took great care that never any human sacrifice should be offered to him. Though he commanded Abraham to offer up his son, yet he would by no means suffer it to be actually done, but appointed something else with which he should be redeemed [Genesis 22]. And though God challenged the firstborn of all living things to be his, yet he appointed that the firstborn of men should be redeemed, and so in all cases wherein persons were holy to the Lord, the law makes provision that they should not be slain, but redeemed. And God, by the prophet Isaiah, declares such sacrifices to be abominable to him in the forementioned Isaiah 66:3. It would have been symbolizing with the abominable customs of the heathen nations that were round, especially that [of] offering human sacrifices to the idol Moloch, which God ever manifested a peculiar detestation of [Leviticus 20:2]. Here particularly observe Deuteronomy 12:2932. And the nature of the case won't allow us to think that Jephthah, in this instance, committed such abomination. 'Tis not likely but that he, being a pious person, as he is spoken of by the Apostle [Hebrews 11:32], would have been restrained from it by God. And then what was done was doubtless agreeable to the mind and will of God, for God otherwise would not, in so extraordinary a manner, have assisted her so quickly and readily to resign herself to it. There seems most evidently an extraordinary divine influence on her mind in the affair, for her resignation did [not] arise from senselessness or indifference of spirit, as is evident, because she desired time so to bewail what was to be done to her. And upon the supposition that she was to be slain, it would be impossible, without an extraordinary influence on her mind, for her to be so resigned. Her resignation was from pious considerations, and holy and excellent principles, as is evident by what she says to her father, when she sees him passionately lamenting the issue of his vow, of which we have an account in the Hebrews 11:36. "And she said unto him, My father, if thou hast opened thy mouth unto the Lord, do to me according to that which hath proceeded out of thy mouth; forasmuch as the Lord hath taken vengeance for thee of thine enemies, even of the children of Ammon."
If what he had vowed to do was so abominable a thing, as to kill her in sacrifice, it would not have been her duty to say, as she does, "do to me according to that which hath proceeded out of thy mouth." But she seemed to be influenced to express herself as she did by the Spirit of God, and her resignation is recorded of her as a very excellent thing in her.
III. Her being to be slain in sacrifice seems inconsistent with her request to go up and down the mountains to bewail her virginity; it would have been rather to bewail her untimely end.
IV. It seems evident that she was not slain, by the Hebrews 11:39. 'Tis said, that "it came to pass that at the end of two months, she returned unto her father, who did with her according to his vow which he had vowed"; and the consequence of it is immediately added, "and she knew no man." This clause seems evidently to be exegetical of the foregoing, viz. that he did to her "according to his vow"; or to explain what that was that he did, viz. devote her to God in a perpetual virginity. If she had been slain, 'tis not at all likely that it would have been mentioned, that "she knew no man," for that she had known no man before this had been already expressed, in her going up and down the mountains to bewail her virginity; and nobody would suppose that she would marry and have children after she was devoted to death, and it had been determined, both by herself and her father, that it should be put in execution. And besides, there would have been no occasion to mention not knowing man, because as soon as the two months was out, wherein she bewailed her virginity, and she had returned from going up and down the mountains, the vow was immediately executed.
V. It is no argument that Jephthah thought himself obliged to put her to death, that he so lamented when his daughter met him, as Hebrews 11:35. "And it came to pass, when he saw her, that he rent his clothes, and said, Alas, my daughter, thou hast brought me very low, and thou art one of them that trouble me; for I have opened my mouth unto the Lord, and I cannot go back." For she, being his only child, by her being devoted to be a Nazarite, his family was entirely extinct. He had no issue to inherit his estate, or keep his name in remembrance, which in those days was looked upon an exceeding great calamity. "Thou hast brought me very low," i.e. thou hast quenched my coal, and brought perpetual barrenness on thyself. See Poole's Synopsis, at the end of Judges 11.