Jesus says of Satan in John’s Gospel, “He was a murderer from the beginning and does not stand in the truth, because there is no truth in him. When he tells a lie, he speaks in character, because he is a liar and the father of lies,” (John 8:44). Later Jesus will say of Himself, “I am the way and the truth and the life. No one comes to the Father except through me,” (John 14:6). The Christian therefore must live in the truth, embracing it and speaking it, lest we not only deny reality, but Christ. When we lie, we speak the language of Hell.
When it comes to oaths, vows, and promises, there is a degree to which we ought not make them at all. Not only, as Jesus points out, is there nothing we could swear upon that does not already belong to God, but since part of being a follower of Christ involves being truthful, we ought not need to make an oath at all: we should be living lives where our word is known to be trustworthy and true.
“A good person out of the store of goodness in his heart produces good, but an evil person out of a store of evil produces evil; for from the fullness of the heart the mouth speaks,” (Luke 6:45).