I have read the account at Judges 11 in its entirety. I see no suggestion of human sacrifice, only a promise to sacrifice. To say that is what ultimately happened here is pure speculation and conjecture. We even have another example of a child given to serve in the temple as a vow, that of Samuel. If we look at the text, it appears that what may have been bewailed was her virginity, not her death. Either way, Jephthah's vow was rash, and is a signal warning.
Besides, the Law was clear at Deuteronomy 18:10: When you enter the land the LORD your God is giving you, do not learn to imitate the detestable ways of the nations there. Let no one be found among you who sacrifices his son or daughter in the fire.
I do not see how Jephthah's vow could anull the Law, and I doubt the Levite priesthood would have sacrificed a human being on the tabernacle altar in direct conflict with it. The penalty for child sacrifice was death by stoning.
E.W.Bullinger (Great Cloud of Witnesses in Hebrew 11 (1911) ISBN 0825422477), looks at the word "and" in the Jephthah’s vow (Judges 11:31: "whatever comes out of the doors of my house to meet me, when I return in peace from the people of Ammon, shall surely be the Lord's, and I will offer it up as a burnt offering"). As he explains [8] the Hebrew word ? that is translated in the above passage as "and" is often used as a disjunctive, and means "or", when there is a second proposition. Indeed this rendering is suggested in the margin of the A.V. Bullinger goes on to give examples from the Bible where the same word has been translated as "or". According to him, the right translation of this passage is: "whatever comes out of the doors of my house to meet me, when I return in peace from the people of Ammon, shall surely be the Lord's, or I will offer it up as a burnt offering." Jephthah’s daughter, being the first that came out of the house, was thus, according to Bullinger, dedicated to God. He also says:
"In any case, it should have been unlawful, and repugnant to Jehovah, to offer a human being to Him as a burnt-offering, for His acceptance. Such offerings were common to heathen nations at that time, but it is noteworthy that Israel stands out among them with this great peculiarity, that human sacrifices were unknown in Israel."
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