Again you have heard that it was said to those of old, ‘You shall not swear falsely, but shall perform to the Master [Lord] what you have sworn.’ But I say to you, Do not take an oath 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 take an oath 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. 
Why this is important: Jesus lives in a clear world. He is light. There is no darkness in Him at all. Our heavenly Father is all about clarity. God demands the same from us. His radical world and plan for us is that we be clear as well. No ambiguity. No waffling. No wavering.
In Jesus’ world, he challenges us to no “ifs, ands or buts”. It is all yes and no. Jesus challenges us to not be manipulative, trying to get our own way with our answers. God’s goal for us is clarity.
A simple “yes” or “no” should be enough for a trustworthy person, a saying of Jesus that James passes on (James 5:12). It is true that Paul invoked an oath because the people did not know him well enough to be certain of his character yet, so we can see that oaths are not disallowed in the New Testament.
Jesus’ point is that a disciple’s simple word should be considered as trustworthy as a signed document or contract. When he goes further to suggest that “anything beyond this is evil,” Jesus indicates that swearing by something in order to deceive can only have one source—the evil one, Satan.
God’s goal: Our word should be trustworthy because we should be trustworthy. Our Master Jesus the Messiah [Christ] shows us the way. Integrity is a huge issue in all our relationships.
 The Holy Bible: English Standard Version. (2016). (Mt 5:33–37). Wheaton, IL: Crossway Bibles.
 Wilkins, M. J. (2004). Matthew (p. 248). Grand Rapids, MI: Zondervan Publishing House.