Jephthah judged and led Israel during his time. At a time when israel was being harassed and ransacked by the Amorites, he led Israel into battle against the enemy who had a grudge against them for taking their land over 300 years back.
A bit of background first. Jephthah was brushed aside by his relatives because he was the son of a prostitute. His relatives were ashamed that a child of a prostitute may lead them some day so they banished him from their land.
But God would use Jephthah in a special way. He was to lead Israel against the Ammonite enemy.
In verse 34, when Israel was about to attack, Jephthah made a vow that he would regret later. He told God that whatever or whoever came out of his house first when he returned triumphant would be sacrificed to the Lord. Even when the Spirit of the Lord was on him, he still wanted an assurance from God.
God gave Jephthah the victory, went home and soon found his beloved and only daughter coming out of the house. Jephthah was mortified as he remembered his vow. This is his only daughter whom he loved so much. But such is the price of a vow made recklessly. I say reckless because there was no mention that he even thought of or prayed about it. What other thing was in his house for him to confidently say such a vow that he would not think that his daughter, wife or any other loved one will not come out. A goat? A servant?
But God gave Jephthah victory and he had to fulfill his vow and lamented it. Jephthah’s vow was that he will sacrifice as a burnt offering the first one who came out of his house when he came home triumphantly. It is not said what happened to his daughter. Since God abhors human sacrifices, it is assumed that his daughter, his only daughter would remain a virgin and not marry. For Israelite women this is like death.
When we ask God for a favor and we offer something in exchange, let us be sure of what we are going into. It might be better if we ask God what He wants, if ever He asks something in return. God is wisdom. What He decides upon is for our good. When we go ahead of Him without knowing His perfect will, like making a vow with such haste, then we are in danger of making a big mistake.