"Command what You will, and then grant what You command."
Or just as often this famous quoute from St. Augustine's Confessons is cited in reversed order: "Grant what You command, and command what You will." Either way, St. Augustine is imploring God's grace in this little prayer. He recognizes two things we also ought to keep in mind.
First, God is God and His will is supreme. He can, and does, command whatever He wills, whatever pleases Him, whatever is right. He does not submit to our wills, although His love for His creatures is reflected in His will. Calvinists are especially fond of referring to His "sovereign" will, and my only reservation is to guard against the idea that "sovereign" here implies anything arbitrary in God's will. No, God's will is congruent and consistent with His own holy nature. He does not will anything that is contrary to His love or justice or purity.
So what does He will for us? He wills us to be righteous, and therein lies the problem. God demands we be right in what we say and do and think. He wills us not merely to act righteous, but to be righteous. But humans are fallen creatures. We are not simply fallible or prone to mistakes. Nor do we merely have trouble keeping God's commands. On our own, we are sinners bent on being our own gods. The corrupt human nature cannot conform itself to God's will, and cannot do any works that are truly, entirely good. This is why, apart from Christ, we are condemned.
But in Christ, we are saved, forgiven, reborn, and made righteous. In Christ, who is our righteousness, we have the clearest revelation of how God can command what He will, can insist His creatures be righteous, and yet, turn around and give to us what He commands. As far as righteousness, He imputes to us, through faith, the righteousness of Christ. Then He pours out on us His Holy Spirit, who begins by converting our wills so that we do want to submit to His will.
Wherever God's grace is at work, His Spirit is actively granting to us as a pure gift, what His sovereign will is perfectly right in commanding of us. First to last, it's God at work within us, doing in us what we cannot do ourselves. Next we'll look at why this teaching caused some in the Church to greatly criticize Augustine over this remark.