De Sacrificiis Abelis Et Caini

Now it is for the sake of obtaining credence that those men who are disbelieved have recourse to an oath. But God is to be believed when simply he says any thing; so that, as far as certainty goes, his words do in no respect differ from oaths. And it happens, indeed, that our opinions are confirmed by an oath; but that an oath itself is confirmed by the addition of the name of God. God, therefore, does not become credible because of an oath, but even an oath is confirmed by God.