Deuteronomy 23:21, 23 When thou shalt vow a vow unto the Lord thy God, thou shalt not slack to pay it; for the Lord thy God will surely require it of thee; and it would be a sin in thee….That which is gone out of thy lips thou shalt keep and perform; even a free will offering, according as thou hast vowed unto the Lord thy God, which thou hast promised with thy mouth.
People often make rash vows and promises in regard to relationships with God or other people. If one makes a promise it is a sin to him if he does not keep it. It is like a lie, but not quite the same. But it is a matter of faithfulness, integrity, and trust. Keeping promises and paying vows is a characteristic of a man or woman of God: he does what he says.
Moses also says in v. 22 that it is not a sin if you do not vow. But not keeping a promise you did make is a sin. Therefore, we are very careful about making vows and promises in uncertain affairs and frivolous matters, for once it has escaped our lips we are bound to it. However, there are promises we do make in serious situations and legal contracts. For instance, it is right to make a marriage vow, and keep it. It is proper to make a Confirmation vow in church. It is required to take an oath in court to tell the truth. It is helpful to make contracts and pay back loans.
The Christian only makes promises he knows he can and will keep. Even when there is no legal penalty to enforce a vow, we have in internal enforcer that holds us to account to keep our promises. So we watch what we say. And we repent; we are not like the son in the parable who said he would work but didn’t. We become faithful and trustworthy because God is faithful to His promises, and we can count on it.