26 Instead of entering on the distinctions between the praiseworthy and the blameworthy, the criminal and the innocent, the dangerous and the harmless, the guilty and the guiltless, the desirable and the undesirable, which are all illustrations of the distinction between sin and righteousness, we must first consider what sin is, and then examine the actions of the saints as recorded in the holy books, that, if we find these saints described as sinning, we may if possible discover the true reason for keeping these sins in memory by putting them on record.
Again, if we find things recorded which, though they are not sins, appear so to the foolish and the malevolent, and in fact do not exhibit any virtues, here also we have to see why these things are put into the Scriptures which we believe to contain wholesome doctrine as a guide in the present life, and a title to the inheritance of the future. As regards the examples of righteousness found among the acts of the saints, the propriety of recording these must be plain even to the ignorant.
The question is about those actions the mention of which may seem useless if they are neither righteous nor sinful, or even dangerous if the actions are really sinful, as leading people to imitate them, because they are not condemned in these books, and so may be supposed not to be sinful, or because, though they are condemned, men may copy them from the idea that they must be venial if saints did them.
Source: Reply to Faustus the Manichaean (New Advent)