3 You cannot distinguish between what God does in mercy and what He does in judgment, because you can neither understand nor use the words of our Psalter: "I will sing of mercy and judgment unto You, O Lord." Accordingly, whatever in the feebleness of your frail humanity seems amiss to you, you separate entirely from the will and judgment of God: for you are provided with another evil god, not by a discovery of truth, but by an invention of folly; and to this god you attribute not only what you do unjustly, but also what you suffer justly.
Thus you assign to God the bestowal of blessings, and take from Him the infliction of judgments, as if He of whom Christ says that He has prepared everlasting fire for the wicked were a different being from Him who makes His sun to rise upon the evil and the good, and sends rain on the just and on the unjust. Why do you not understand that this great goodness and great severity belong to one God, but because you have not learned to sing of mercy and judgment? Is not He who causes the sun to rise on the evil and the good, and sends rain on the just and on the unjust, the same who also breaks off the natural branches, and engrafts contrary to nature the wild olive tree?
Does not the apostle, in reference to this, say of this one God: "You see, then, the goodness and severity of God: to them which were broken off, severity; but toward you, goodness, if you continue in His goodness?" Here it is to be observed how the apostle takes away neither judicial severity from God, nor free-will from man. It is a profound mystery, impenetrable by human thought, how God both condemns the ungodly and justifies the ungodly; for both these things are said of Him in the truth of the Holy Scriptures.
But is the mysteriousness of the divine judgments any reason for taking pleasure in cavilling against them? How much more becoming, and more suitable to the limitation of our powers, to feel the same awe which the apostle felt, and to exclaim, "O the depth of the riches both of the wisdom and of the knowledge of God! How unsearchable are His judgments, and His ways past finding out!" How much better thus to admire what you cannot explain, than to try to make an evil god in addition to the true God, simply because you cannot understand the one good God! For it is not a question of names, but of actions.
Source: Reply to Faustus the Manichaean (New Advent)