65 The impiety, therefore, of Faustus' attacks on Scripture can injure no one but himself; for what he thus assails is now deservedly the object of universal reverence. As has been said already, the sacred record, like a faithful mirror, has no flattery in its portraits, and either itself passes sentence upon human actions as worthy of approval or disapproval, or leaves the reader to do so. And not only does it distinguish men as blameworthy or praiseworthy, but it also takes notice of cases where the blameworthy deserve praise, and the praiseworthy blame.
Thus, although Saul was blameworthy, it was not the less praiseworthy in him to examine so carefully who had eaten food during the curse, and to pronounce the stern sentence in obedience to the commandment of God. So, too, he was right in banishing those that had familiar spirits and wizards out of the land. And although David was praiseworthy, we are not called on to approve or imitate his sins, which God rebukes by the prophet. And so Pontius Pilate was not wrong in pronouncing the Lord innocent, in spite of the accusations of the Jews; nor was it praiseworthy in Peter to deny the Lord thrice; nor, again, was he praiseworthy on that occasion when Christ called him Satan because, not understanding the things of God, he wished to withhold Christ from his passion, that is, from our salvation.
Here Peter, immediately after being called blessed, is called Satan. Which character most truly belonged to him, we may see from his apostleship, and from his crown of martyrdom.
Source: Reply to Faustus the Manichaean (New Advent)