God orders what we cannot do, that we may know what we ought to ask of him. There is a great utility in precepts [commands], if all that is given to free will is to do greater honor to divine grace. Faith acquires what the law requires; nay, the requires, in order that faith may acquire what is thus required; nay, more, God demands of us faith itself, and finds not what he thus demands, until by giving he makes it possible to find it.Augustine is saying that God demands what we cannot do because we are sinful and enslaved. But, his demanding makes way for his grace, for by his grace faith acquires what we in our sinfulness cannot - righteousness, obedience to God's precepts. In fact, Augustine pushes it further, even the faith God requires he finds lacking in us until he bestows it upon us a free gift. Thus, in all of this, God is magnified as the Holy God who demands righteousness of his people and the Gracious God who gives to his people what he cannot find in them. Beautiful!
Calvin also includes the famous prayer of Augustine, which would be an appropriate prayer for us all to learn as we take up Bob's challenge to pray before we act: "Grant what you command and command what you will."