73 According to the eternal law, which requires the preservation of natural order, and forbids the transgression of it, some actions have an indifferent character, so that men are blamed for presumption if they do them without being called upon, while they are deservedly praised for doing them when required. The act, the agent, and the authority for the action are all of great importance in the order of nature. For Abraham to sacrifice his son of his own accord is shocking madness.
His doing so at the command of God proves him faithful and submissive. This is so loudly proclaimed by the very voice of truth, that Faustus, eagerly rummaging for some fault, and reduced at last to slanderous charges, has not the boldness to attack this action. It is scarcely possible that he can have forgotten a deed so famous, that it recurs to the mind of itself without any study or reflection, and is in fact repeated by so many tongues, and portrayed in so many places, that no one can pretend to shut his eyes or his ears to it.
If, therefore, while Abraham's killing his son of his own accord would have been unnatural, his doing it at the command of God shows not only guiltless but praiseworthy compliance, why does Faustus blame Moses for spoiling the Egyptians? Your feeling of disapproval for the mere human action should be restrained by a regard for the divine sanction. Will you venture to blame God Himself for desiring such actions? Then "Get behind me, Satan, for you understand not the things which be of God, but those which be of men."
Would that this rebuke might accomplish in you what it did in Peter, and that you might hereafter preach the truth concerning God, which you now, judging by feeble sense, find fault with! As Peter became a zealous messenger to announce to the Gentiles what he objected to at first, when the Lord spoke of it as His intention.
Source: Reply to Faustus the Manichaean (New Advent)