Judges 11:39

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.

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.