He had not yet accepted God's grace, and submitted to God. Before he was converted he said: "the power of willing is the power of doing; and as yet I could not do it. Thus my body more readily obeyed the slightest wish of the soul in moving its limbs at the order of my mind than my soul obeyed itself to accomplish in the will alone its great resolve" (10.VI.20). When his spiritual will truly accepted Christ, his body followed and God freed him from unwanted desire. He accepted his lack of ability to master his body, and accepted that he needed grace to be good.
Thus although he speaks of 'the body' and 'the spirit,' these things are not two separate entities -- in fact, Augustine condemns the Manichean notion of "two wills" and the notion that the world is evil in its physical essence (10.X.22). The world and the human body, after all, are created by God, and God is good. Augustine's conception of the will is that God must urge to soul to accept Him, and God is always there, but the heart must be open enough to hear God's call to grace. The human soul must freely choose God, even while the soul's perfection and restraint is dependant upon God's grace. If the spirit is truly willing, then the body easily follows. This is seen when Augustine takes up a book, upon the urging of a divine voice and reads the scripture that is the mechanism for his final conversion: "I heard the voice of a boy or a girl...
The voice of the child for Augustine is the voice of Christ.
Ultimately, Augustine's conversion seems more of an example along the lines of the Lutheran model of conversion and salvation, that man is saved by divine grace alone, not by his physical actions in the world, despite Augustine's revered place in the Catholic pantheon of saints. Merely rejecting physical cravings is not enough, and to focus only on mastering the body is not the true path to salvation. For the physical will to be changed and overcome, the spiritual and internal must be changed, and a divine force is required for this change. Then, once a real, internal shift occurs, the physical alteration of behavior seems to happen naturally, as it did for Augustine. Once Augustine really prayed with a true heart after his conversion, not merely a false heart to 'become good, but not quite yet' Augustine was reformed. This suggests that there are not 'two wills,' as the Manicheans falsely believed, but that a sinning body is evidence of a sinning soul, even though the sinner may half-heartedly want to be good -- but not yet.
Works Cited
Augustine. "Confessions." Christian Classics Ethereal Library. 11 Nov 2008. http://www.ccel.org/ccel/augustine/confessions.xi.html
