奥古斯丁的著作《上帝之城》的写作背景是什么？在《上帝之城》中，奥士阐述了怎样的思想？

<kungfu fighting>

Everybody was kung-fu fighting
Those cats were fast as lightning
In fact it was a little bit frightning
But they fought with expert timing

They were funky China men from funky Chinatown
They were chopping them up and they were chopping them down
It's an ancient Chineese art and everybody knew their part
From a feint into a slip, and kicking from the hip

Everybody was kung-fu fighting
Those cats were fast as lightning
In fact it was a little bit frightning
But they fought with expert timing

There was funky Billy Chin and little Sammy Chung
He said here comes the big boss, lets get it on
We took a bow and made a stand, started swinging with the hand
The sudden motion made me skip now we're into a brand knew trip

Everybody was kung-fu fighting
Those cats were fast as lightning
In fact it was a little bit frightni