But I say unto you, Swear not at all; neither by heaven; for it is God’s throne: Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. Neither shalt thou swear by thy head, because thou canst not make one hair white or black. But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil. Matthew 5:34-37
But above all things, my brethren, swear not, neither by heaven, neither by the earth, neither by any other oath: but let your yea be yea; and your nay, nay; lest ye fall into condemnation. James 5:12
And with regard to our not swearing at all, and always speaking the truth, He commanded as follows: “Swear not at all; but let your yea be yea, and your nay, nay; for whatsoever is more than these comes of evil.” Justin Martyr (A.D. 160) Ante-Nicene Fathers vol.1 pg.168
The man of proved character in such piety is far from being apt to lie and to swear. For an oath is a decisive affirmation, with the taking of the divine name. For how can he, that is once faithful, show himself unfaithful, so as to require an oath; and so that his life may not be a sure and decisive oath? He lives, and walks, and shows the trustworthiness of his affirmation in an unwavering and sure life and speech. Clement of Alexandria (A.D. 195) Ante-Nicene Fathers vol.2 pg.537
But he does not even swear, preferring to make averment, in affirmation by “yea,” and in denial by “nay.” For it is an oath to swear, or to produce anything from the mind in the way of confirmation in the shape of an oath. It suffices, then, with him, to add to an affirmation or denial the expression “I say truly,” for confirmation to those who do not perceive the certainty of his answer. For he ought, I think, to maintain a life calculated to inspire confidence towards those without, so that an oath may not even be asked; and towards himself and those with whom he associates? Good feeling, which is voluntary righteousness.Clement of Alexandria (A.D. 195) Ante-Nicene Fathers vol.2 pg.537
Where, then, is the necessity for an oath to him who lives in accordance with the extreme of truth? He, then, that does not even swear will be far from perjuring himself… And so he swears not even when asked for his oath; nor does he ever deny, so as to speak falsehood, though he should die by tortures. Clement of Alexandria (A.D. 195) Ante-Nicene Fathers vol.2 pg.537-538
In borrowing money from heathens under pledged securities, Christians give a guarantee under oath, and deny themselves to have done so… .Christ prescribes that there is to be no swearing.Tertullian (A.D. 198) Ante-Nicene Fathers vol.3 pg. 75
Of perjury I am silent since even swearing is not lawful. Tertullian (A.D. 198) Ante-Nicene Fathers vol.3 pg. 67
But in order that we may not by saying everything at once today, confuse your minds, we put before you one custom, namely, about the avoidance of oaths, saying this much by way of preface, and speaking plainly — that if you do not avoid oaths, I say not perjury merely, but those too which happen in the cause of justice, we shall not further discourse upon any other subject…. For this error is grave, and it is exceedingly grave because it does not seem to be grave, and on this account I fear it, because no one fears it. On this account the disease is incurable, because it does not seem to be a disease; but just as simple speech is not a crime, so neither does this seem to be a crime, but with much boldness this transgression is committed: and if any one call it in question, straightway laughter follows, and much ridicule, not of those who are called in question for their oaths, but of those who wish to rectify the disease. On this account I largely extend my discourse about these matters. For I wish to pull up a deep root, and to wipe out a long-standing evil: I speak not of perjury alone, but even of oaths in good faith. “But so and so,” says one, “a forbearing man, consecrated to the priesthood, living in much self-control and piety, takes an oath.” Do not speak to me of this forbearing person, this self-controlled, pious man who is consecrated to the priesthood; but if you will, add that this man is Peter, or Paul, or even an angel descended out of heaven. For not even in such a case do I regard the dignity of their persons. For the law which I read upon oaths, is not that of the servant, but of the King: and when the edicts of a king are read, let every claim of the servants be silent. But if you are able to say that Christ bade us use oaths, or that Christ did not punish the doing of this, show me, and I am persuaded. But if he forbids it with so much care, and takes so much thought about the matter as to class him who takes an oath with the evil one (for whatsoever is more than these, namely, than yea and nay, says he, is of the devil), Matthew 5:35 why do you bring this person and that person forward? For not because of the carelessness of your fellow servants, but from the injunctions of his own laws, will God record his vote against you. I have commanded, he says, you ought to obey, not to shelter yourself behind such and such a person and concern yourself with other persons’ evil…. For our reckoning is not with our fellow-servants, but with our Master, and to him we shall give account for all done in our life. Let us prepare ourselves therefore for this tribunal. For even if he who transgresses this law be beyond everything revered and great, he shall certainly pay the penalty attaching to the transgression. For God is no respecter of persons. – John Chrysostom, Instructions to Catechism, First Introduction