Jephthah (Judges 11 NASB) didn't have it easy. Although he lived with his father's family, his mother was "a harlot" (perhaps a pagan temple prostitute). When his father's two legitimate sons grew older, they kicked Jephthah out. Jephthah fled and gathered around him a gang of outcasts and rejects. But apparently these "worthless fellows" were good in a fight. When Ammon started attacking Israel, Jephthah's half-brothers called him back: "Come and be our chief that we may fight against the sons of Ammon." Jephthah negotiated his terms: "If you take me back to fight against the sons of Ammon and the Lord gives them up to me, will I become your head?" They agreed.
Must we always keep our vows/oaths to God? Is it better to break an oath or fulfill an unwise or sinful vow?
Jephthah was a good leader, but he didn't want to attempt such a big endeavor on his own. He needed God's approval. But instead of seeking out a judge—or just asking—he made a foolish vow: "If You will indeed give the sons of Ammon into my hand, then it shall be that whatever comes out of the doors of my house to meet me when I return in peace from the sons of Ammon, it shall be the Lord's and I will offer it up as a burnt offering." Jephthah was probably thinking one of his sheep or goats, housed on the ground floor, would come out. Instead, it was his daughter.
Several hundred years later, King Saul (1 Samuel 14) was so frustrated with his battle against the Philistines that he sent out the order: "Cursed be the man who eats food before evening, and until I have avenged myself on my enemies." Besides giving a really foolish command, Saul didn't even make sure his entire army got the news. He didn't realize that his son, Jonathan, and Jonathan's armor-bearer had slipped away to attack the garrison on their own. Israel's main army saw the commotion and joined the fight.
Meanwhile, Jonathan was hungry. He found a bee hive on the battlefield and ate a little honey. God withdrew His counsel from Saul, and Saul rightly figured it was because someone had disobeyed his order. The lots fell to Jonathan who offered himself to be executed.
It isn't clear whether Jephthah really killed his daughter or just dedicated her to serve in God's tabernacle—she mourns for her virginity, not her life. But he did sacrifice her. Although Saul was prepared to kill his son, the people around them intervened: "Must Jonathan die, who has brought about this great deliverance in Israel? Far from it! As the Lord lives, not one hair of his head shall fall to the ground, for he has worked with God this day" (1 Samuel 14:45 NASB). The ambiguity begs the question—if you have made a foolish or even sinful vow to God, is it worse to break the vow or commit the sin?
In the culture of Bible times, honor was a serious matter. A man's reputation was the most important thing he owned. To renege on a vow to God was a serious offence. The Bible has several things to say about oaths.
Numbers 30:2: "If a man vows a vow to the Lord, or swears an oath to bind himself by a pledge, he shall not break his word. He shall do according to all that proceeds out of his mouth."
Leviticus 5:4: "or if anyone utters with his lips a rash oath to do evil or to do good, any sort of rash oath that people swear, and it is hidden from him, when he comes to know it, and he realizes his guilt in any of these."
Deuteronomy 23:21-23: "If you make a vow to the LORD your God, you shall not delay fulfilling it, for the LORD your God will surely require it of you, and you will be guilty of sin. But if you refrain from vowing, you will not be guilty of sin. You shall be careful to do what has passed your lips, for you have voluntarily vowed to the LORD your God what you have promised with your mouth."
Proverbs 20:25: "It is a snare to say rashly, 'It is holy,' and to reflect only after making vows."
Matthew 5:33-37: "Again you have heard that it was said to those of old, 'You shall not swear falsely, but shall perform to the Lord what you have sworn.' But I say to you, Do not take an oath at all, either by heaven, for it is the throne of God, or by the earth, for it is his footstool, or by Jerusalem, for it is the city of the great King. And do not take an oath by your head, for you cannot make one hair white or black. Let what you say be simply 'Yes' or 'No'; anything more than this comes from evil."
For the most part, the Mosaic Law instructs us to fulfill our vows. Proverbs 20:25 says it's the height of foolishness to make a vow without understanding the implications, and in Matthew 5, Jesus says it's better to just not give an oath at all.
Leviticus 5:4 (NASB) is interesting, however. First of all, the vow or oath is "with his lips"—that is, verbal and official. For us, it would be swearing an oath in front of witnesses or signing an affidavit or contract. This does not include mental thoughts we tell ourselves. The vow in question is either impossible to fulfill or would require the person to sin (to do evil). The phrase "it is hidden from him, and then he comes to know about it" means that he forgot and was reminded. "He will be guilty in one of these things" means that he is guilty either of swearing rashly ("speak thoughtlessly") or intending to do evil.
The passage does not say that a person must sin or do the impossible to fulfill a vow. It says if a person vows something that turns out to be impossible or sinful, he is guilty of swearing rashly or intending to do harm. These are sins, and there may be repercussions, but it is better to take the sin of carelessness or malice rather than fill an oath by sinning.
If the oath is possible and can be filled without sinning, however, it should be fulfilled. This may cause a great deal of discomfort or hardship, but our position as representatives of Christ is more important than our worldly position.
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