Matthew 5:34-37

"But I tell you, Do not swear at all: either by heaven, for it is God's throne; 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 even one hair white or black.  Simply let your 'Yes' be 'Yes' and your 'No' No; anything beyond this comes from the evil one."

Here, Jesus was emphasizing the importance of telling the truth.   People were breaking promises and using sacred language casually and carelessly.  Keeping oaths and promises is important; it build trust and makes committed human relationships possible.  The bible condemns making vows or taking oaths casually, giving your word while knowing that you won't keep it or swearing falsely in God's name.  Oaths are needed in certain situations only because we live in a sinful society that breeds distrust. 

Oaths or vows were common, but Jesus told his followers not to use them - their word alone should be enough.  Are you known as a person of your word?  Truthfulness seems so rare that we feel we must end out statements with "I promise".  If we tell the truth all the time, we will have less pressure to back up our words with an oath or promise.