Jephthah made a vow to the LORD and said, “If You will indeed give the sons of Ammon into my hand, then it shall be that whatever comes out of the doors of my house to meet me when I return in peace from the sons of Ammon, it shall be the LORD’S, and I will offer it up as a burnt offering.” So Jephthah crossed over to the sons of Ammon to fight against them; and the LORD gave them into his hand. He struck them with a very great slaughter from Aroer to the entrance of Minnith, twenty cities, and as far as Abel-keramim. So the sons of Ammon were subdued before the sons of Israel. When Jephthah came to his house at Mizpah, behold, his daughter was coming out to meet him with tambourines and with dancing. Now she was his one and only child; besides her he had no son or daughter. When he saw her, he tore his clothes and said, “Alas, my daughter! You have brought me very low, and you are among those who trouble me; for I have given my word to the LORD, and I cannot take it back.” So she said to him, “My father, you have given your word to the LORD; do to me as you have said, since the LORD has avenged you of your enemies, the sons of Ammon.” She said to her father, “Let this thing be done for me; let me alone two months, that I may go to the mountains and weep because of my virginity, I and my companions.” Then he said, “Go.” So he sent her away for two months; and she left with her companions, and wept on the mountains because of her virginity. At the end of two months she returned to her father, who did to her according to the vow which he had made; and she had no relations with a man. Thus it became a custom in Israel, that the daughters of Israel went yearly to commemorate the daughter of Jephthah the Gileadite four days in the year.
What Jephthah vowed he would do with his daughter was no different than what Herod promised he would do with John the Baptist. The Bible is silent regarding God condoning the vow that Jephthah made or the burnt offering. It was a foolish vow since he had no idea what he would be offering and if he had any guts he would have taken his daughter’s place.
4-Point Calvinist, Ron Rhodes, comments: “There are a few interpretative options. One view is that Jephthah actually did offer his own daughter as a burnt sacrifice to the Lord. If this is the case, this does not in any way mean that God endorsed what Jephthah did.” (Commonly Misunderstood Bible Verses, p.56, emphasis mine)