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.
[AD 397] Ambrose of Milan on Judges 11:39-40