Judges 9-12 (The Daily Walk Bible).

Did Jephthah really sacrifice his daughter? After a long discussion, here is the conclusion from Hard Sayings of the Bible: “Jephthah was acquainted with the law of Moses that forbade human sacrifice. Judges 11:12–28 shows he knew the history of Israel and could recite it at will. But this, of course, is no proof that what he knew he always did, any more than our knowing what is right guarantees we will always do it. That Jephthah actually sacrificed his daughter, tragic as that would be, seems the most natural reading of the text. If Jephthah’s ‘sacrifice’ of his daughter meant relegating her to a life of perpetual virginity and service at the temple, not one word in the text says so. The only possible support is the comment that whoever comes out of the house ‘will be the LORD’s’ (Judg 11:31). But the statement immediately after this proves he had a whole burnt offering in mind—’sacrifice … as a burnt offering.'”

As far as oaths go today, Jesus said this: “Let what you say be simply ‘Yes’ or ‘No’; anything more than this comes from evil” (Matthew 5:37). And, in thinking about keeping our promises, Hard Sayings also has this to say about oaths, which I think is very practical advice for making promises as well: “Oaths or vows that violate a moral law of God, however, should not be kept.”