Again, you have heard that it was said to the people long ago, ‘Do not break your oath, but fulfill to the Lord the vows you have made.’ But I tell you, do not swear an oath 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. All you need to say is simply ‘Yes’ or ‘No’; anything beyond this comes from the evil one.
Jesus’ instruction in today’s passage focuses on vows and oaths. A vow is a promise that takes place between man and God or between two individuals. An oath is a declaration that adds collateral for weightiness. In an oath I swear by something or someone greater than myself. When someone takes an oath in court they swear to tell the truth, the whole truth, and nothing but the truth — so help them God (by someone greater than themselves).
It was the habit of religious leaders in Jesus’ day to take the truth and twist it. They taught that whether or not an oath was binding depended on the wording. They taught that a promise wasn’t binding if you did not swear by God’s name. Kent Hughes explains, “Evasive swearing became a fine art. The height of accomplishment was, while lying, to convince another person you were telling the truth.” Jesus said to cut it out! Be a man or woman of your word. Don’t lose your integrity by playing with words. All you need to say is “Yes” or “No.” 
The same applies today. Let your “Yes” be “Yes.” Let your “No” be “No.” Be a person of integrity. Be a person of your word.
Father, don’t let me play with the truth. Help me speak clearly and plainly with integrity. Help me be a person of my word. In Jesus’ name. Amen.
 Kent R. Hughes, Faith that Works (Wheaton, Ill.: Crossway Books, 1991).