Jesus calls us to be people of the truth and people who keep their word. We need to be people whose yes is yes and whose no is no.
"Again you have heard that it was said to those of old, 'You shall not swear falsely, but shall perform to the Lord what you have sworn.' 34 But I say to you, Do not take an oath at all, either by heaven, for it is the throne of God, 35 or by the earth, for it is his footstool, or by Jerusalem, for it is the city of the great King. 36 And do not take an oath by your head, for you cannot make one hair white or black. 37 Let what you say be simply 'Yes' or 'No'; anything more than this comes from evil.