Ambrose of Milan
I maintain that the military leader Jephthah foolishly made a promise, as noted in Judges 11:30-31. St. Ambrose expands upon this narrative in his work 'On Virginity' (2. 5-7), where he narrates how Jephthah vowed to sacrifice to the Lord whatever first greeted him upon his return home, ultimately regretting that vow when he encountered his daughter at the doorstep. In his sorrow, he tore his garments and exclaimed, "Alas, my daughter! Because of you I am in a grievous predicament. You have been the cause of my grief" (Judg. 11:35). Although he carried out his harsh judgment out of a sense of religious duty, he also established an annual mourning period for his family, instructing them to grieve for her.
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The vow itself was cruel, and the act of fulfilling it was even more so, leading him to lament the decision he made. This mourning became a tradition among the Israelites, as every year the daughters of Israel would commemorate the daughter of Jephthah the Gileadite for four days (Judg. 11:40). While I cannot fault Jephthah for feeling obligated to uphold his oath, it is indeed a tragic duty that came at the cost of his beloved child's life. It would be wiser to refrain from making vows than to commit to something that neither the deity addressed nor the recipient desires to fulfill. We remember the example of Isaac, for whom the Lord permitted a ram to be offered instead (Gen. 22:13); thus, not every promise must be kept. The Lord Himself frequently alters His decisions, as evidenced in the Scriptures. In the Book of Numbers, He contemplated bringing death upon the people but later relented at Moses' intercession (Num. 14:12). Additionally, He instructed Moses and Aaron to separate from the assembly, proclaiming that He would swiftly eliminate them (Num. 16:21). Following their departure, the earth opened up, engulfing the rebellious Dathan and Abiron.