The story of Jephthah is told in Judges: Israel is being invaded and the people are afraid. But help is available: "Then the spirit of the LORD came upon Jephthah, and he passed over Gilead, and Manasseh, and passed over Mizpeh of Gilead, and from Mizpeh of Gilead he passed over unto the children of Ammon. And Jephthah vowed a vow unto the LORD, and said, If thou shalt without fail deliver the children of Ammon into mine hands, Then it shall be, that whatsoever cometh forth of the doors of my house to meet me, when I return in peace from the children of Ammon, shall surely be the LORD’S, and I will offer it up for a burnt offering. So Jephthah passed over unto the children of Ammon to fight against them; and the LORD delivered them into his hands. And he smote them from Aroer, even till thou come to Minnith, even twenty cities, and unto the plain of the vineyards, with a very great slaughter. Thus the children of Ammon were subdued before the children of Israel. And Jephthah came to Mizpeh unto his house, and, behold, his daughter came out to meet him with timbrels and with dances: and she was his only child; beside her he had neither son nor daughter. And it came to pass, when he saw her, that he rent his clothes, and said, Alas, my daughter! thou hast brought me very low, and thou art one of them that trouble me: for I have opened my mouth unto the LORD, and I cannot go back. And she said unto him, My father, if thou hast opened thy mouth unto the LORD, do to me according to that which hath proceeded out of thy mouth; forasmuch as the LORD hath taken vengeance for thee of thine enemies, even of the children of Ammon. And she said unto her father, Let this thing be done for me: let me alone two months, that I may go up and down upon the mountains, and bewail my virginity, I and my fellows. And he said, Go. And he sent her away for two months: and she went with her companions, and bewailed her virginity upon the mountains. And it came to pass at the end of two months, that she returned unto her father, who did with her according to his vow which he had vowed: and she knew no man. And it was a custom in Israel, That the daughters of Israel went yearly to lament the daughter of Jephthah the Gileadite four days in a year" (Judges 11: 29-40).

What troubles Protestant commentators about this story is Jephthah's vow. St. Paul in the Epistle to the Hebrews fits Jephthah into a series beginning with Abel of those called to God by faith: "Through faith we understand that the worlds were framed by the word of God, so that things which are seen were not made of things which do appear.  By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he was righteous, God testifying of his gifts: and by it he being dead yet speaketh"  (Hebrews 11:4-5) and moving on to the great Judges of Israel: "By faith the walls of Jericho fell down, after they were compassed about seven days.  By faith the harlot Rahab perished not with them that believed not, when she had received the spies with peace. And what shall I more say? for the time would fail me to tell of Gedeon, and of Barak, and of Samson, and of Jephthae; of David also, and Samuel, and of the prophets:  Who through faith subdued kingdoms, wrought righteousness, obtained promises, stopped the mouths of lions,  Quenched the violence of fire, escaped the edge of the sword, out of weakness were made strong, waxed valiant in fight, turned to flight the armies of the aliens" (Hebrews 11:31-35). Yet Calvin insists that even those who have received the gift of faith are not thereby made into perfect human beings: "Everything praiseworthy that they did the Apostle has attributed to faith, although there was none of them whose faith did not falter. . . . Samson was the victim of the enticements of his mistress and thoughtlessly betrayed the safety of himself and of all his people. Jephtheh rushed headlong into making a foolish vow and was over-obstinate in performing it, and thereby marred a fine victory by the cruel death of his daughter. In every saint there is always to be found something reprehensible. Nonetheless although faith may be imperfect and incomplete it does not cease to be approved by God. There is no reason, therefore, why the fault from which we labor should break us or discourage us provided we go on by faith in the race of our calling" (David W. Torrance and Thomas F. Torrance, ed. Calvin's Commentaries: The Epistle of Paul the Apostle to the Hebrews and the First and Second Epistles of St. Peter, trans. William B. Johnston [Grand Rapids: Wm. B. Eerdmans, 1963, 1980), p. 182). Similarly, in the marginal note to Judges 11: 29-40, the commentator[s] suggest that "As the Apostle comendeth Iphath for his worthy enterprise in delivering the people, by his rashe vowe and wicked performance of the same, his victorie was defaced, and here we seethat the sinnes of the godly do not utterly extinguish their faith." It is possible as well that contemporary audiences at Hamlet might have seen in Hamlet's calling Polonius "old Jephthah" (II.ii.404-22) someone who has been pondering the making of rash vows himself, as he will shortly say in his soliloquy (II.ii.600-605).