Again, you have heard that the ancients were told, ‘YOU SHALL NOT MAKE FALSE VOWS, BUT SHALL FULFILL YOUR VOWS TO THE LORD.’ But I say to you, make no oath at all, either by heaven, for it is the throne of God, or by the earth, for it is the footstool of His feet, or by Jerusalem, for it is THE CITY OF THE GREAT KING. Nor shall you make an oath by your head, for you cannot make one hair white or black. But let your statement be, ‘Yes, yes’ or ‘No, no’; anything beyond these is of evil. Matthew 5:33-37
Listen to the simplicity as Jesus cut to the chase regarding the reason behind oaths or vows in the first place. The reason? Because there is no real harmony between man's words and his intentions. People believe their words really have the power to bring their claims to fruition, and yet as Jesus said, a person cannot make one insignificant thing happen simply by speaking it. In truth, the vow, for man, is all about creating some wiggle room. In other words, men love to make promises and then later claim that there were valid reasons for not keeping it. The bottom line is that people will say whatever needs to be said to get something in return.
The reason Jesus told them to not even make vows at all was not to go against the Law but to uncover the sneakiness of man (for the Law is not bad, but good). It does not take any special revelation to understand that if people were to simply mean what they said ('Yes, yes' or 'No, no') there would be no reason for a vow ... for it is common knowledge that a vow is meant to hold a man to his word, in case he later decide to break it. And even then, through the accumulation of lawyers and new rulings, people have discovered that they can easily lose what is owed them, or end up owing what they never agreed to. This is the result of the mind of man as he twists the Law to his own benefit.
Now, I know there are many who have set Jesus' words up as some kind of new covenant command, but this is a total perversion of what he meant. I'm not saying that we should not do what we say, I'm saying that the truth behind what Jesus said is found within the grace of God that was to come through his own body and blood. For he was in the process of bringing about the new creation in himself. We have difficulty with all this because we are caught up in the failure of the flesh, which we keep alive by living as if we are still under Law. It's past time we discovered the truth about who we are and what we are truly about.
Therefore from now on we recognize no one according to the flesh; even though we have known Christ according to the flesh, yet now we know Him in this way no longer. Therefore if anyone is in Christ, he is a new creature; the old things passed away; behold, new things have come. 2 Corinthians 5:16-17