From "Nicene and Post-Nicene Fathers Series 1, Volume 5: St Augustine - Anti-Pelagian Writings", ed. Phillip Schaff, Christian Classics Ethereal Library.
Now there is much significance in that He does not say, “The wrath of God shall come upon him,” but “abideth on him.” For from this wrath (in which we are all involved under sin, and of which the apostle says, “For we too were once by nature the children of wrath, even as others”) nothing delivers us but the grace of God, through Jesus Christ our Lord. The reason why this grace comes upon one man and not on another may be hidden, but it cannot be unjust. For “is there unrighteousness with God? God forbid.” But we must first bend our necks to the authority of the Holy Scriptures, in order that we may each arrive at knowledge and understanding through faith. For it is not said in vain, “Thy judgments are a great deep.” The profundity of this “deep” the apostle, as if with a feeling of dread, notices in that exclamation: “O the depth of the riches both of the wisdom and the knowledge of God!” He had indeed previously pointed out the meaning of this marvellous depth, when he said: “For God hath concluded them all in unbelief, that He might have mercy upon all.” Then struck, as it were, with a horrible fear of this deep: “O the depth of the riches both of the wisdom and the knowledge of God! how unsearchable are His judgments, and His ways past finding out! For who hath known the mind of the Lord? or who hath been His counsellor? or who hath first given to Him, and it shall be recompensed unto him again? For of Him, and through Him, and in Him, are all things: to whom be glory for ever. Amen.” How utterly insignificant, then, is our faculty for discussing the justice of God’s judgments, and for the consideration of His gratuitous grace, which, as men have no prevenient merits for deserving it, cannot be partial or unrighteous, and which does not disturb us when it is bestowed upon unworthy men, as much as when it is denied to those who are equally unworthy!