"But I say to you, do not swear at all...instead, let your Yes be Yes, and your No be No...for...anything else is from the devil." Swearing, or taking an oath, is involving, in fact too involving that Jesus doesn't want his followers to get themselves involved in swearing or taking oaths. Swearing involves witnesses. Those who swear by God make God their witness. Those who swear by any part of their body make those body parts their witnesses. The same applies to swearing by heaven, earth, Jerusalem, or any other thing out there by which people swear. Jesus says, "none of these belongs to you. None of these is yours. If you have to swear, swear with what is yours! I think the reason is very simple: speak the truth! Be honest. Be trustworthy. If what comes out of my mouth is truthful and honest, I have no need for a witness. Let the truth be my sole witness. Let me swear by the truth that I speak. And the truth doesn't mean that it has to be Yes all the time! Jesus himself knows that there will be times when we have to say No. And it is okay, as long as it is the truth. Sometimes we lie by saying yes to please others. It is this lie that Jesus says comes from the devil. Be truthful. Be honest. Do not give the devil any room.