33 Again, when Faustus accuses a righteous and faithful man of a shameless profanation of his marriage from avarice and greed, by selling his wife Sara at different times to the two kings Abimelech and Pharaoh, telling them that she was his sister, because she was very fair, he does not distinguish justly between right and wrong, but unjustly condemns the whole transaction. Those who think that Abraham sold his wife cannot discern in the light of the eternal law the difference between sin and righteousness; and so they call perseverance obstinacy, and confidence presumption, as in these and similar cases men of wrong judgment are wont to blame what they suppose to be wrong actions.
Abraham did not become partner in crime with his wife by selling her to others: but as she gave her handmaid to her husband, not to gratify his passion, but for the sake of offspring, in the authority she had consistently with the order of nature, requiring the performance of a duty, not complying with a sinful desire; so in this case, the husband, in perfect assurance of the chaste attachment of his wife to himself, and knowing her mind to be the abode of modest and virtuous affection, called her his sister, without saying that she was his wife, lest he himself should be killed, and his wife fall into the hands of strangers and evil-doers: for he was assured by his God that He would not allow her to suffer violence or disgrace.
Nor was he disappointed in his faith and hope; for Pharaoh, terrified by strange occurrences, and after enduring many evils on account of her, when he was informed by God that Sara was Abraham's wife, restored her with honor uninjured. Abimelech also did the same, after learning the truth in a dream.
Source: Reply to Faustus the Manichaean (New Advent)