One of the reasons Judges can be a baffling book is that the author of Hebrews 11 mentions a number of figures as part of the "hall of fame" for people of faith. In his by now standard commentary on Hebrews, William Lane noted that the phrase that can be rendered "enacted justice" referred to the establishment of just governance In his NICOT commentary on the book of Judges Barry Webb notes that the judges were generally military leaders who delivered Israel from oppression or judicial figures who governed.
Webb makes a broader point about Gideon and Jephthah that is important to stress, that in both cases men who were clothed with the spirit or full of the spirit of the Lord nevertheless did not lose their peculiar personality traits. To be blunt, having a full dose of the spirit of the Lord doesn't mean Gideon stopped being cowardly or that Jepthah takes a prudent vow. Both men open with little speeches that start with "if".
Let's take a look at Gideon's incident in Judges 6:36-40 (NIV)
Gideon said to God, “If you will save Israel by my hand as you have promised— 37 look, I will place a wool fleece on the threshing floor. If there is dew only on the fleece and all the ground is dry, then I will know that you will save Israel by my hand, as you said.” 38 And that is what happened. Gideon rose early the next day; he squeezed the fleece and wrung out the dew—a bowlful of water.
39 Then Gideon said to God, “Do not be angry with me. Let me make just one more request. Allow me one more test with the fleece, but this time make the fleece dry and let the ground be covered with dew.” 40 That night God did so. Only the fleece was dry; all the ground was covered with dew.
Since it was customary for nomadic tribes and those in lands without freshwater streams to collect water by leaving fleece to collect dew overnight Barry Webb cites a scholar or two in his commentary on Judges (ISBN 978-0-8028-2628-2, page 239) to show that Gideon's first test of using a fleece intimates that not only is Gideon a bit of a coward but also a bit of a moron. Webb's introduction to the book takes note of how blunt and even crude humor in the Book of Judges frequently is. Wenatchee The Hatchet finds the incident of the fleece funny. Gideon asks for the fleece to have water and for the ground to be dry but then realizes this is what NORMALLY HAPPENS and since he is not really confident Yahweh will really be with him, he asks that Yahweh submit to an actually difficult and conclusive test. He's aware enough that putting God to the test could get him in trouble, but Gideon does it anyway.
The punchline here is that Gideon does this after being clothed with the spirit. This might be comparable to Peter confessing Jesus was the Christ and then being called "Satan" in Matthew 16. Clearly one moment of divinely enabled inspiration and utterance does not preclude immediately being followed by a terrible act or statement.
Now, let's get to Jephthah's vow.
29Then the Spirit of the Lord came on Jephthah. He crossed Gilead and Manasseh, passed through Mizpah of Gilead, and from there he advanced against the Ammonites. 30And Jephthah made a vow to the Lord: “If you give the Ammonites into my hands, 31whatever comes out of the door of my house to meet me when I return in triumph from the Ammonites will be the Lord’s, and I will sacrifice it as a burnt offering.”
Something simple and even obvious that Webb points out that a reader might miss is that God does not at any point address Jephthah. Gideon gets some fairly direct assurance. Jepthah does not. In fact Webb goes so far as to suggest that in the narrative that though the spirit of God comes upon Jepthhah it is not clear at all that Jephthah is aware this has happened. If he was why did he make the vow?
It is important to not read the preamble of Judges as though it applied across the board to all the narratives in Judges. For instance, in Judges 10 we're shown the Lord said He would not deliver Israel from trouble.
11The Lord replied, “When the Egyptians, the Amorites, the Ammonites, the Philistines, 12the Sidonians, the Amalekites and the Maonitesc oppressed you and you cried to me for help, did I not save you from their hands? 13But you have forsaken me and served other gods, so I will no longer save you. 14Go and cry out to the gods you have chosen. Let them save you when you are in trouble!”
15But the Israelites said to the Lord, “We have sinned. Do with us whatever you think best, but please rescue us now.” 16Then they got rid of the foreign gods among them and served the Lord. And he could bear Israel’s misery no longer.
17When the Ammonites were called to arms and camped in Gilead, the Israelites assembled and camped at Mizpah. 18The leaders of the people of Gilead said to each other, “Whoever will take the lead in attacking the Ammonites will be head over all who live in Gilead.”
God takes pity on the Israelites after telling them He would not save them and inviting them to cry out to their gods instead. God rejects their overtures of repentance as fraudulent.
When Jephthah becomes a judge, as Webb stress (page 313) it is a human appointment and not a divine one. There is no angel of the Lord appointing Jephthah. Jephthah is cast out from his people and becomes a warrior with no social status in the tribe. It is only when a dire military situation emerges that, as some kind of last resort, his kinsmen remember him. As with Yahweh, so with Jephthah, remembered only after all other persons and options were tried and wanting. In a fascinating parallel narrative as Yahweh rejects the appeal of Israelites Jephthah does, too. The new judge agrees to fight on the condition that he is made a judge and, of course, in order to gain and keep that status he has to win the battle. If we don't soak ourselves in the narrative and its details it can be easy to over look how much hedging of bets happens in this narrative and Webb makes a persuasive case in his commentary that Jephthah's rash vow was not impulsive but, if anything, vague, calculating and hedged. In case readers don't remember the vow and the man's reaction to his daughter, recall that the man blames his daughter for bringing him very low.
An argument against the impulsiveness of the vow can be proposed, per Webb, from the sheer detail and length of the pre-war correspondence Jephthah was willing to go through to actually avoid a full-blown military confrontation. The lengthy parlay also could have provided time to muster enough troops to go to battle if battle was inevitable. In a sweep from pages 300 to 330 Barry Webb's commentary on Judges illustrates a series of bribes and bargains made by Israelites to Yahweh and then Jephthah and how Yahweh could no longer tolerate the oppression of Israel and let Jephthah, the outcast and warrior, fight to save his people. But Jephthah's vow is itself ultimately a bribe, in Webb's opinion, made by a man who can't bring himself to believe God will deliver him or his people. Some kind of offering had to be made to sweeten the deal and guarantee results. Webb points out that in terms of the Hebrew there is no room for any doubt that the offering would be a burnt offering. What is vague, even evasive, is the thing to be offered. Webb goes so far as to suggest that like Israel Jephthah is willing to pledge loyalty and an offering to the Lord but only what is forced from him. When his daughter meets him after the victory Jephthah puts all the blame on her for bringing him low rather than on himself for making the vow he has made.
So what, then, would have made Jephthah a hero of the faith? In his commentary on Hebrews Lane suggested that Jephthah trusted God would deliver His people and risked his life to defend them. That seems like a start but I would suggest we consider the possibility that for Jephthah to even bother fighting on behalf of his countrymen after they ostracized and rejected him would be a sign of loyalty to God but also a willingness to risk his fortunes for his people, even if he insisted in exchange he be made a ruler if he won. In a society that cast out the illegitimately born Jephthah had everything to lose if he lost and something to gain if he won, but was the people pledging themselves conditionally to him worth the trouble? That was, too, an act of faith.
Webb's commentary on the vow and its fulfillment take as given that the daughter was offered as a burnt offering. Webb points out that unlike the sacrifice of a child in Genesis this one in Judges was not commanded of the Lord, but it also makes it impossible to propose that it was never possible for God to require a human sacrifice as a matter of categorical prohibition. Maybe God wanted to see if Abraham would follow through. Webb makes a more detailed case for why the infamous vow was evasive and calculating and why Jephthah should not have kept it but for that you'd best go buy the book (ISBN previously mentioned).