78 It is therefore mere groundless calumny to charge Moses with making war, for there would have been less harm in making war of his own accord, than in not doing it when God commanded him. And to dare to find fault with God Himself for giving such a command, or not to believe it possible that a just and good God did so, shows, to say the least, an inability to consider that in the view of divine providence, which pervades all things from the highest to the lowest, time can neither add anything nor take away; but all things go, or come, or remain according to the order of nature or desert in each separate case, while in men a right will is in union with the divine law, and ungoverned passion is restrained by the order of divine law; so that a good man wills only what is commanded, and a bad man can do only what he is permitted, at the same time that he is punished for what he wills to do unjustly.
Thus, in all the things which appear shocking and terrible to human feebleness, the real evil is the injustice; the rest is only the result of natural properties or of moral demerit. This injustice is seen in every case where a man loves for their own sake things which are desirable only as means to an end, and seeks for the sake of something else things which ought to be loved for themselves. For thus, as far as he can, he disturbs in himself the natural order which the eternal law requires us to observe.
Again, a man is just when he seeks to use things only for the end for which God appointed them, and to enjoy God as the end of all, while he enjoys himself and his friend in God and for God. For to love in a friend the love of God is to love the friend for God. Now both justice and injustice, to be acts at all, must be voluntary; otherwise, there can be no just rewards or punishments; which no man in his senses will assert. The ignorance and infirmity which prevent a man from knowing his duty, or from doing all he wishes to do, belong to God's secret penal arrangement, and to His unfathomable judgments, for with Him there is no iniquity.
Thus we are informed by the sure word of God of Adam's sin; and Scripture truly declares that in him all die, and that by him sin entered into the world, and death by sin. And our experience gives abundant evidence, that in punishment for this sin our body is corrupted, and weighs down the soul, and the clay tabernacle clogs the mind in its manifold activity; and we know that we can be freed from this punishment only by gracious interposition. So the apostle cries out in distress, "O wretched man that I am!
Who shall deliver me from the body of this death? The grace of God through Jesus Christ our Lord." So much we know; but the reasons for the distribution of divine judgment and mercy, why one is in this condition, and another in that, though just, are unknown. Still, we are sure that all these things are due either to the mercy or the judgment of God, while the measures and numbers and weights by which the Creator of all natural productions arranges all things are concealed from our view.
For God is not the author, but He is the controller of sin; so that sinful actions, which are sinful because they are against nature, are judged and controlled, and assigned to their proper place and condition, in order that they may not bring discord and disgrace on universal nature. This being the case, and as the judgments of God and the movements of man's will contain the hidden reason why the same prosperous circumstances which some make a right use of are the ruin of others, and the same afflictions under which some give way are profitable to others, and since the whole mortal life of man upon earth is a trial, who can tell whether it may be good or bad in any particular case— in time of peace, to reign or to serve, or to be at ease or to die— or in time of war, to command or to fight, or to conquer or to be killed?
At the same time, it remains true, that whatever is good is so by the divine blessing, and whatever is bad is so by the divine judgment.
Source: Reply to Faustus the Manichaean (New Advent)