Augustine was born in a small town in North Africa in the mid-300s AD. His parents were not poor, so when he got older and it seemed that he was very smart, they sent him to a bigger city to go to school. Eventually they sent him to Rome to study with teachers there. At first Augustine was interested in Manichaeanism. While he was studying in Italy as a young man, he met Ambrose, the bishop of Milan. Ambrose (and the general enthusiasm in Rome) convinced Augustine to convert to Christianity. Soon afterwards Augustine went back to Africa to be the bishop of Hippo, a town near his hometown. He wrote a book, the Confessions, about why he had converted to Christianity.
Not long after Augustine returned home, he got the terrible news that the city of Rome had been attacked and sacked by the Visigoths (in 410 AD). Augustine was shocked and sad. Then he heard that people were saying that Rome had been destroyed because the Romans had converted to Christianity and forgotten the old gods. People were saying that they should go back to praying to Jupiter and Mars, and then Rome would be safe.
Augustine was very upset by this. No, it could not be so! He knew that the Christian god was the one true god, and Jupiter and Mars were only demons. But then why had Rome been destroyed, just as everyone was finally converting to Christianity?
Augustine devoted most of the rest of his life to writing a book, the City of God, that would answer this question. His answer (in a very short version) was that God wanted the things of this earth to be destroyed so that the new world could come. The City of Man (Rome) was destroyed to make things ready for the City of God (Heaven) which was going to come very soon. Meanwhile, everybody should convert to Christianity quickly, or else when the world ended they would go to hell.
Augustine’s view helped everyone to deal with the fact that the Roman Empire was collapsing around them, and became very popular.
Augustine died in 430 AD, with the Vandals besieging Hippo, the city he lived in. But he died happy, knowing that when the Vandals took Hippo, it was only another sign that the Kingdom of God was coming soon.
Learn by doing: read the beginning of the City of God
More about the Vandals