In Matthew 5, Jesus strongly condemns the sins of the tongue - extending to the smallest detail and intent. He warns against unfriendly words, words that fail to show hospitality, words that raise suspicion, words that condemn a brother, and even words of "harmless slander" that can ultimately have a hurtful end.
He reserved his strongest statements for his condemnation of oath-taking.
Again, you have heard that it was said to those of ancient times, 'You shall not swear falsely, but carry out the vows you have made to the Lord.' But I say to you, Do not swear at all, either by heaven, for it is the throne of God, 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 one hair white or black. Let your word be 'Yes, Yes' or 'No, No'; anything more than this comes from the evil one. (Matthew 5:33-37 NRSV)
These sayings of Jesus focus not only on oath-taking in legal proceedings (courtrooms and contracts), but to a more general constant and consistent attitude of truth-telling which manifests itself in the everyday life of his followers. Participants in the kingdom of God should have such reputations as truth-tellers that no one should expect or demand any act of heroic assertion of their truthfulness.
When one's "Yes" means "Yes" and "No" means "No", there is no need to swear one's honesty by heaven or earth. Oaths - exaggerated protestations of one's truthfulness - are only required of those who are expected to lie. Those living under God's rule need make no such affirmations.
Jeremias concludes, "God is the God of truth, and therefore truth is a characteristic of his reign."