6 Do not swear. Swearing is not profanity. Swearing is to take an oath, or to swear "to tell the truth." Swearing is forbidden by Jesus and James: Above all things, my brothers, do not swear, neither by heaven or by earth, nor by any other oath; but let your "Yes" be yes; and your "No," no, to avoid being condemned. James 5:12. The reason Jesus further tells us not to swear by heaven, by earth, (or by the Bible), by God, or even the hairs on your head is because you don't control any of them, so to pretend to be able invoke them is a complete exaggeration, even a lie.
But let your 'yes' be 'yes,' and your 'no,' 'no;' for whatever is more than these comes from the evil one. Until our selfish spirit, the spirit of Satan, that every man has, is crucified, most of what we speak is from that Spirit of Satan; even if you have "accepted Jesus," your mouth is still controlled by the spirit of Satan until it is crucified on the inward cross of self-denial to be replaced by the Spirit of Christ to rule your heart in thoughts, words, deeds. Until you heart has been purged of the evil in it, by the powerful grace of God, to become purified, your words come from the Spirit of Satan. That is why Jesus did not want man to bear witness about him, because he knew what was in their hearts. John 2:25. And many other times, Jesus ordered men not to talk about the miracles he had done; for he did not want the testimony of men with their hearts still full of evil, as we all are, until cleansed by Him. That is why Jesus said: But I say to you, for every idle [ineffective] word that men speak, they will give an account of the same in the day of judgment. Mat 12:36
Man's heart is full of evil; man's mind is enmity to God. What an uncrucified man speaks is from his evil heart, mind, and spirit; that is why the Lord must give us a new heart, new mind, and new spirit as we must become an entirely new creature, with all old things passed away. That is why a disciple is blind until Jesus, the Master, makes the disciple perfect like Jesus is perfect; to be able to help the disciple's brother remove the spec from his eye. Luke 6:39-45