Quotes about oaths and swearing from throughout Christian History.
The man of proven 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? Shouldn't his life be a sure and decisive oath? He lives, walks, and shows the trustworthiness of his affirmation in an unwavering and sure life and speech.
... But he does not even swear, preferring to affirm, in the positive 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's sufficient, then, for him to add to an affirmation or denial the expression, "I'm telling the truth," 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 outside, so that an oath may not even be asked; and towards himself and those with whom he associates good feeling, which is voluntary righteousness.
The gnostic [not meaning the religion, but as Clement uses it, the devout Christian] does swear, but is not apt to swear, having rarely recourse to an oath, just as we have said. And his speaking truth on oath arises from his accord with the truth. This speaking truth
on oath, then, is found to be the result of correctness in duties. Where, then, is the necessity for an oath to him who lives in accordance with the extreme of truth?
He, then, that does not ever swear will be far from perjuring himself. And he who does not transgress in what is ratified by agreements will never swear, since the ratification of the violation and of the fulfillment is by actions ... he who lives justly, transgressing in none of his duties ... swears truth by his action. As a result, testimony by the tongue is in his case superfluous.
Therefore, persuaded always that God is everywhere, and fearing not to speak the truth, and knowing that it is unworthy of him to lie, he is satisfied with the divine consciousness and his own alone. And so he lies not, nor does he do anything contrary to his agreements. And so he doesn't swear even when asked for his oath; nor does he ever deny, so as to speak falsehood, though he should die by tortures. (Miscellanies VII:8)