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.”
Clement of Alexandria
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.
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. -ibid
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. -ibid
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.
Of perjury I am silent since even swearing is not lawful.- ibid