From: Matthew 5:33-37
Jesus and His Teaching, the Fulfillment of the Law (Continuation)
(Jesus said to His disciples,)  "Again you have heard that it was said to the men of old, `You shall not swear falsely, but shall perform to the Lord what you have sworn.'  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 what you say be simply, `Yes' or `No'; anything more than this comes from evil."
33-37. The Law of Moses absolutely prohibited perjury or violation of oaths (Exodus 20:7; Numbers 30:3; Deuteronomy 23:22). In Christ's time, the making of sworn statements was so frequent and the casuistry surrounding them so intricate that the practice was being grossly abused. Some rabbinical documents of the time show that oaths were taken for quite unimportant reasons. Parallel to this abuse of oath-taking there arose no less ridiculous abuses to justify non-fulfillment of oaths. All this meant great disrespect for the name of God. However, we do know from Sacred Scripture that oath-taking is lawful and good in certain circumstances: "If you swear, `As the Lord lives', in truth, in justice, and in uprightness, then nations shall bless themselves in Him, and in Him shall they glory (Jeremiah 4:2).
Jesus here lays down the criterion which His disciples must apply in this connection. It is based on re-establishing, among married people, mutual trust, nobility and sincerity. The devil is "the father of lies" (John 8:44). Therefore, Christ's Church must teach that human relationships cannot be based on deceit and insincerity. God is truth, and the children of the Kingdom must, therefore, base mutual relationships on truth. Jesus concludes by praising sincerity. Throughout His teaching He identifies hypocrisy as one of the main vices to be combatted (cf., e.g., Matthew 23:13-32), and sincerity as one of the finest virtues (cf. John 1:47).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.