For the Christian, failure to keep a vow or promise or covenant or contract is not merely a moral or ethical issue but a Gospel issue. This notion is on the mind of the apostle Paul when he pivots upon Galatians 3:15 to demonstrate that believers can rest their full weight on the Gospel because it stands upon the promises God made to Abraham — promises that can never be set aside or amended.
To give a human example, brothers: even with a man-made covenant, no one annuls it or adds to it once it has been ratified. (Galatians 3:15 ESV)
We live in a culture that rests upon many Biblical values, so even non-Christians expect people to keep their agreements. Faithfulness in contractual agreements and covenants is a universal practice that, in the words of Paul, literally no one would think of setting aside by annulment or subversion through amendments that render an agreement meaningless or absurd. Commenting on this verse, Leon Morris points out that covenant faithfulness is crucial not only between God and man but between average people in a healthy society:
Paul is pointing out that a covenant is a binding engagement. A duly ratified covenant means that the parties have come to an agreement and the covenant expresses the terms of that agreement. When that has been done no-one nullifies or alters the terms agreed upon. If such alterations were possible, human affairs would be thrown into complete confusion. It is of the first importance that binding agreements be honored.
Our heavenly Father keeps His promises. If we are his children, we will be careful not to annul or add to those vows or contracts we make with Him or with one another.