Augustine of Hippo, On Rebuke and Grace Chapter 37: To a Sound Will is Committed the Power of Persevering or of Not Persevering

As, therefore, the first man did not receive this gift of God,—that is, perseverance in good, but it was left in his choice to persevere or not to persevere, his will had such strength,—inasmuch as it had been created without any sin, and there was nothing in the way of concupiscence of himself that withstood it,—that the choice of persevering could worthily be entrusted to such goodness and to such facility in living well. But God at the same time foreknew what he would do in unrighteousness; foreknew, however, but did not compel him to this; but at the same time He knew what He Himself would do in righteousness concerning him. But now, since that great freedom has been lost by the desert of sin, our weakness has remained to be aided by still greater gifts. For it pleased God, in order most effectually to quench the pride of human presumption, “that no flesh should glory in His presence”—that is, “no man.” But whence should flesh not glory in His presence, save concerning its merits? Which, indeed, it might have had, but lost; and lost by that very means whereby it might have had them, that is, by its free will; on account of which there remains nothing to those who are to be delivered, save the grace of the Deliverer. Thus, therefore, no flesh glories in His presence. For the unrighteous do not glory, since they have no ground of glory; nor the righteous, because they have a ground from Him, and have no glory of theirs, but Himself, to whom they say, “My glory, and the lifter up of my head.” And thus it is that what is written pertains to every man, “that no flesh should glory in His presence.” To the righteous, however, pertains that Scripture: “He that glorieth, let him glory in the Lord.” For this the apostle most manifestly showed, when, after saying “that no flesh should glory in His presence,” lest the saints should suppose that they had been left without any glory, he presently added, “But of Him are ye in Christ Jesus, who of God is made unto us wisdom, and righteousness, and sanctification, and redemption: that, according as it is written, He that glorieth, let him glory in the Lord.” Hence it is that in this abode of miseries, where trial is the life of man upon the earth, “strength is made perfect in weakness.” What strength, save “that he that glorieth should glory in the Lord”?