Nothing so far is to say that the City of God is full of perfect people--"even good men can be sick, suffering from that disobedience... which is the penalty of a primal disobedience which, therefore, is a wound or weakness in a nature that is good in itself."
Thus, we have to be "growing in grace and living by faith during [our] pilgrimage on earth." This is why God has given us His Word, to help us walk through this world growing in holiness and grace. This is the "medicine" we receive "during [our] pilgrimage on earth while praying for the peace of [our] heavenly fatherland"--that along with the holy Spirit, who works within this internal medicine of the Word.
All told, Augustine has a wonderful doctrine of sanctification on display here--we hear the Word preached externally, and it is applied to our hearts by the Holy Spirit: "Even though God makes use of one of His obedient creatures [the preacher]... it is only by His interior grace that He moves and rules our mind."
This is how, as we pass through this world, we can begin to tell citizens of the Heavenly city from citizens of the worldly one. Christians pass through this world with God at work in their hearts, yearning more and more for an increase in the application of the Word and the arrival of the peace of heaven, either by our getting there or by Christ's return.
Nor is this to say that the city of man is left without a word from God. Even Cain was warned, and he sinned anyway, both in his bad sacrifice and in his refusal to heed God's warning, and of course in the murder of his brother.
Here we have the very heart of the earthly city. Its God (or gods) is he or they who will help the city to victory after victory and to a reign of earthly peace; and this city worships, not because it has any love for service, but because its passion is for domination. This, in fact, is the difference between good men and bad men, that the former make use of the world in order to enjoy God, whereas the latter would like to make use of God in order to enjoy the world--if, of course, they believe in God and His providence over man, and are not so bad as those who deny even this.What Cain should have done, Augustine says, is sacrificed again, this time the right way: "he should have made a change in himself in order to imitate his brother." Instead, he worshiped himself by giving in to sin: first envy, then anger, then finally rejection of God's Word and murder. Cain gave in to his passions and fleshly appetites rather than submitting to God. This desire to serve our own flesh is likewise what God curses Eve with, and "was the founder of the city of earth."