“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)
Christ tells us to make no oath or vow at all, neither a false vow nor a true vow. Why? Because making a vow or an oath is exercising a work, depending on a man-made construct for doing good. I want to keep my word because my heart is truthful, not because I made a vow and I am superstitiously bound to the vow, whether I want to fulfill it or not. Oaths an vows force us to do the right thing, even when our hearts are not desiring to do so. The Sermon on the Mount overall tells Christ's disciples what the attributes of a true believer are. So, he tells them, "you don't want to make any vow, because you will be putting your faith in the vow." You want to put your faith in Christ, and so when you do so, Christ changes your heart to one pleasing to God. You don't need to keep a vow, because you are in a new covenant, where your yes is yes and your no is no.
Let's see this in action:
From that time Jesus began to show His disciples that He must go to Jerusalem, and suffer many things from the elders and chief priests and scribes, and be killed, and be raised up on the third day. Peter took Him aside and began to rebuke Him, saying, “God forbid it, Lord! This shall never happen to You.” But He turned and said to Peter, “Get behind Me, Satan! You are a stumbling block to Me; for you are not setting your mind on God’s interests, but man’s.” (Matthew 16:21-23)
What is "God forbid it?" It is a vow that Peter is not able to keep. He is in the covenant of works.
Now Peter was sitting outside in the courtyard, and a servant-girl came to him and said, “You too were with Jesus the Galilean.” But he denied it before them all, saying, “I do not know what you are talking about.” When he had gone out to the gateway, another servant-girl saw him and *said to those who were there, “This man was with Jesus of Nazareth.” And again he denied it with an oath, “I do not know the man.” A little later the bystanders came up and said to Peter, “Surely you too are one of them; for even the way you talk gives you away.” Then he began to curse and swear, “I do not know the man!” And immediately a rooster crowed. And Peter remembered the word which Jesus had said, “Before a rooster crows, you will deny Me three times.” And he went out and wept bitterly. (Matthew 26:69-75)
Peter denied Christ with an oath. Yes, Peter is still in the covenant of works, and he doesn't get out of such until Christ gives him the Holy Spirit. Here is one last example:
Now Jesus stood before the governor, and the governor questioned Him, saying, “Are You the King of the Jews?” And Jesus said to him, “It is as you say.” (Matthew 27:11)
No oath, no vow. Jesus' yes is "yes." Do you see the difference? Through the Sermon on the Mount, Christ sets up clear delineations between the covenant of works and the covenant of grace:
1. Works: do not commit murder. Grace: your heart does not anger.
2. Works: do not commit adultery. Grace: your heart does not lust.
3. Works: do not make false vows. Grace: your heart is honest.
4. Works: repay crimes with equal punishments. Grace: your heart forgives.
5. Works: Love your neighbor and hate your enemy. Grace: your heart loves your enemy.
When Jesus moves us from one covenant to the other, our hearts change. Peter's heart changed when Jesus moved him. The covenant of grace is a covenant of love. Those in such do not get angry, they do not lust, they are honest, they forgive, and they pray for those who persecute them. They also know that Jesus' death on the cross was crucial for the salvation of the world. They don't deny Christ, and they willingly die for each other. When one is in the covenant of grace, Christ is transforming him into a new person. It takes our lives, and we stumble and fall often, but our hearts know we are being changed. Which covenant are you in? Do you look at the cross and see failure, or do you look at the cross and see the highest love?