Albert Barnes Commentary Judges 11:31

"then it shall be, that whatsoever cometh forth from the doors of my house to meet me, when I return in peace from the children of Ammon, it shall be Jehovah`s, and I will offer it up for a burnt-offering." — Judges 11:31 (ASV)

The words of this verse prove conclusively that Jephthah intended his vow to apply to human beings, not animals: for only one of his household could be expected to come forth from the door of his house to meet him. They also preclude any other meaning than that Jephthah contemplated a human sacrifice. This need not, however, surprise us when we recall his Syrian birth and long residence in a Syrian city, where such fierce rites were probably common. The Syrians and Phoenicians were conspicuous among the ancient pagan nations for human sacrifices, and the transfer, under such circumstances, to Yahweh of the rites with which the false gods were honored, is just what one might expect.

The circumstance of the Spirit of the Lord coming on Jephthah (Judges 11:29) is no difficulty; as it by no means follows that because the Spirit of God endowed him with supernatural valor and energy for vanquishing the Ammonites, He therefore also endowed him with spiritual knowledge and wisdom. The Spirit of the Lord came upon Gideon, but that did not prevent his erring in the matter of the ephod (Judges 8:27). Compare 1 Corinthians 12:4–11; Galatians 2:11–14.