Question: "What does the Bible say about keeping your vows / oaths?"
Answer: There are about 30 biblical references to vows, most of which are from the Old Testament. The books of Leviticus and Numbers have several references to vows in relation to offerings and sacrifices. There were dire consequences for the Israelites who made and broke vows, especially vows to God.
The story of Jephthah illustrates the foolishness of making vows without understanding the consequences. Before leading the Israelites into battle against the Ammonites, Jephthah—described as a mighty man of valor—made a rash vow that he would give to the Lord whoever first came out of doors to meet him if he returned home as the victor. When the Lord granted him victory, the one who came out to meet him was his daughter. Jephthah remembered his vow and offered her to the Lord (Judges 11:29-40). Whether or not Jephthah should have kept this vow is dealt with in another article. What this account shows us is the foolishness of rash vows.
Perhaps this is why Jesus gave a new commandment concerning vows. "Again, you have heard that it was said to the people long ago, 'Do not break your oath, but keep the oaths you have made to the Lord.' But I tell you, Do not swear at all: either by heaven, for it is God's throne; or by the earth, for it is his footstool; or by Jerusalem, for it is the city of the Great King. And do not swear by your head, for you cannot make even one hair white or black. Simply let your 'Yes' be 'Yes,' and your 'No ,' 'No'; anything beyond this comes from the evil one" (Matthew 5:33-37).
The principle here is clear for Christians: do not make vows, either to the Lord or to one another. First, we are unable to know for sure whether we will be able to keep vows. The fact that we are prone to the errors in judgment which are part of our fallen nature means that we may make vows foolishly or out of immaturity. Further, we don’t know what the future will bring—only God does. We don’t know what will happen tomorrow (James 4:14), so to make a vow that we will do or not do something is foolish. God is the one in control, not us, and He “works all things together for good for those who love Him and are called according to His purpose” (Romans 8:28). Knowing this, we can see that it is unnecessary to make vows and that it indicates a lack of trust in Him. Finally, Jesus commands that our word be sufficient without making vows. When we say “yes” or “no,” that’s exactly what we should mean. Adding vows or oaths to our words opens us up to the influence of Satan whose desire is to trap us and compromise our Christian testimony.
If we have made a vow foolishly and realized we cannot or should not keep it, we should confess it to God, knowing that He is “faithful and just and will forgive us our sins and purify us from all unrighteousness.” A broken vow, while serious, is not an unforgivable matter if taken to the Lord in true confession. God will not hold us to vows made imprudently, but He expects us to obey Jesus and refrain from making vows in the future.