Smart people can do stupid things. Jephthah was not a stupid or foolish man, but he made a foolish and stupid vow to the Lord. God did not ask him to make any such vow, but his faith in God was weak, and his desire for victory was great enough that he sought to influence God by making a dramatic offer of sacrifice.
In his desperation he vowed to do something that was very common among his enemies. They practiced human sacrifices, but the law of Moses specifically forbids such acts. We can never please God by promising to do what God’s law forbids us to do.
Before we judge Jephthah too harshly, we must remember he grew up among Israelites who were practicing the pagan religions of the nations around them. They were observing and partaking in the pagan rituals of the people who were persecuting them. He had been raised by a generation that no longer knew God, nor taught their children the laws of God. He did not intentionally make an evil vow, but his promise to God was sinful none the less.
The same is often true today. Many people who know God is real, and even believe that God wants what’s best for them, have trouble believing that God will give them victory over their adversary. Some people after being defeated so often by their addictions, habits, fears, depression and even human enemies become desperate. Even though they believe in God, they have trouble really believing God is going to deliver them. Like Jephthah they make foolish vows in an attempt to gain God’s favor. God loves us enough that he let his son Jesus die for us. What sacrifice could we even dream of making that could compare to that love?
God did not want Jephthah to make a sacrifice; He wanted him to be obedient. When the spirit of the Lord came on him, Jephthah advanced, and God gave him overwhelming victory. Obedience, not a promise of sacrifice, leads to victory.