As going to battle was a serious matter, we should not assume Jephthah made his vow rashly and thoughtlessly.
There were probably syncretistic elements in Jephthah’s religious attitude and thinking: his worship of the Lord mixed up with pagan elements. This is in line with the tendency toward idolatry rampant in Israel in the days of the judges. Jephthah had also spent some time abroad, in the company of worthless fellows
(Judges 11:3). His vow, though made to the Lord, appears to imply such a pagan element, namely a trade-off: a high prize—the victory in battle—will require a high sacrifice.
30 And Jephthah made a vow to the LORD and said, “If you will give the Ammonites into my hand,