It is also sometimes contrary to duty to fulfill a promise or to keep an oath. As was the case with Herod, who swore that whatever was asked he would give to the daughter of Herodias, and so allowed the death of John, that he might not break his word. And what shall I say of Jephthah, who offered up his daughter in sacrifice, she having been the first to meet him as he returned home victorious; whereby he fulfilled the vow which he had made that he would offer to God whatever should meet him first. It would have been better to make no promise at all than to fulfill it in the death of his daughter.
Never shall I be led to believe that the leader Jephthah made his vow otherwise than without thought, when he promised to offer to God whatever should meet him at the threshold of his house on his return. For he repented of his vow, as afterwards his daughter came to meet him. He tore his clothes and said, “Alas, my daughter, you have entangled me, you have become a source of trouble for me.” And though with pious fear and reverence he took upon himself the bitter fulfillment of his cruel task, yet he ordered and left to be observed an annual period of grief and mourning for future times. It was a hard vow, but far more bitter was its fulfillment, while he who carried it out had the greatest cause to mourn. Thus it became a rule and a law in Israel from year to year, as it says: “that the daughters of Israel went to lament the daughter of Jephthah the Gileadite four days in a year.” I cannot blame the man for holding it necessary to fulfill his vow, but yet it was a wretched necessity which could only be solved by the death of his child.…What, then, in the case of esteemed and learned people is full of marvel, that in the case of a virgin is found to be far more splendid, far more glorious, as she says to her sorrowing father, “Do to me according to that which has proceeded out of your mouth.” But she asked for a delay of two months in order that she might go about with her companions upon the mountains to bewail fitly and dutifully her virginity now given up to death. The weeping of her companions did not move her, their grief did not prevail upon her, nor did their lamentations hold her back. She did not allow the day to pass, nor did the hour escape her notice. She returned to her father as though returning according to her own desire, and of her own will [she] urged him on when he was hesitating, and acted thus of her own free choice, so that what was at first an awful chance became a pious sacrifice.
For Jephthah likewise, when he had promised that the first thing that met him, after a victorious battle, he would sacrifice, fell into the snare of child murder; for his daughter first meeting him, he sacrificed her, and God did not forbid it. And I know, indeed, that many of the unbelievers impugn us of cruelty and inhumanity on account of this sacrifice; but I should say that the concession in the case of this sacrifice was a striking example of providence and clemency; and that it was in care for our race that he did not prevent that sacrifice. For if after that vow and promise he had forbidden the sacrifice, many also who were subsequent to Jephthah, in the expectation that God would not receive their vows, would have increased the number of such vows, and proceeding on their way would have fallen into child murder. But now, by suffering this vow to be actually fulfilled, he put a stop to all such cases in the future. And to show that this is true, after Jephthah’s daughter had been slain, in order that the calamity might be always remembered and that her fate might not be consigned to oblivion, it became a law among the Jews that the virgins assembling at the same season should bewail during forty days the sacrifice which had taken place; in order that renewing the memory of it by lamentation, they should make all people wiser for the future; and that they might learn that it was not after the mind of God that this should be done, for in that case he would not have permitted the virgins to bewail and lament her. And that what I have said is not conjectural, the event demonstrated; for after this sacrifice, no one vowed such a vow to God. Therefore also he did not indeed forbid this; but what he had expressly commanded in the case of Isaac, that he directly prohibited, plainly showing through both cases that he does not delight in such sacrifices.
As regards to the fact that Jephthah sacrificed his daughter to God as a whole burnt offering, these are the facts: he had vowed that if he were to obtain the victory, he would offer as a whole burnt offering whoever would come out of his house and meet him; because he had vowed this and won the battle and his daughter had been the one to meet him first, he fulfilled his vow. This event has become a great and rather difficult question to settle both for some who investigate the matter with piety and genuinely seek to know what this passage means and for some who out of ignorant impiety oppose the Holy Scriptures and call this a horrible misdeed that the God of the law and prophets would have delighted in sacrifices, yes, even human sacrifices. First let us reply to their calumnies by noting that the whole burnt offerings of cattle did not delight the God of the law and the prophets—or as I prefer to say, the God of Abraham, Isaac and Jacob. What pleased God about those sacrifices was that they were full of meaning and a foreshadowing of future things. We, however, have the very substance which was foreshadowed by these sacrifices that he wished to commend to us. Moreover, there was also a very pertinent reason why those sacrifices have been changed so that they no longer are commanded but even forbidden: it is so that we may not think that God is pleased by such sacrifices according to some carnal passion.
But we rightly ask whether human sacrifices must have foreshadowed future events.… But if this were true, this type of sacrifice would not displease God. But in fact the Scriptures themselves clearly testify that human sacrifices displease him. For when God wanted and commanded all the firstborn to belong to him and to be his, he nonetheless wanted the firstborn of humankind to be redeemed by their parents, so that they would not entrust their firstborn children to God through immolation.…Now clearly God loves and rewards those sacrifices when a just man endures injustice and struggles for the truth even to the point of death or when he is killed by enemies whom he has offended for righteousness’ sake, as he has returned them good for evil, that is, love instead of hatred.… In imitation of Abel, thousands of martyrs have struggled for the truth to the point of death and have been sacrificed by savage enemies. The Scripture says of them, “God has tested them like gold in a furnace and he has accepted them as a whole burnt offering.” So too the apostle says, “I am being sacrificed.”13
But that is not how Jephthah made a whole burnt offering to the Lord out of his daughter. Rather he offered her as a literal sacrifice in the way that it was commanded for animals to be offered and forbidden for humans to be sacrificed. What he did seems rather similar to what Abraham did. In that instance the Lord gave him a special command that this ought to be done. He did not order him by way of a general commandment that such sacrifices should take place at some time. Indeed, the general rule prohibited it.
[AD 397] Ambrose of Milan on Judges 11:39-40