This is a gem of a book. Short enough to read in an afternoon, but so packed full of ideas and primary source material that you will return to it repeatedly in your research.
Chapter 1 details the "triumphalist" approach to understanding the suppossed triumph of Christianity ofver paganism, a la Eusebius, but that the defeat of Adrianople shook the beleif in an ordered and understandable "god on our side" worldview. According to Brown, Augustine had a more sober view of reality, which was pessimistic about "this age", and which eventually overtook the former idealism. Augustine laments the encroachment of pagan practices as converts streamed in, akin to Cato the Elder's lament about foreigners in Rome.
Chapter 2 explores the intorlerance for alternate theologies and beliefs in the Theodosian Age, a truly sad chapter in Christian history (IMHO). What is refreshing to me is how Brown points out that in the midst of anti-Semitism and anti-Jewish activites there was a greater than expected (or currently understood and taught) amount of civility and respect. Remember, keeping the empire together was their main priority. So it was the upper class, moreso than the church or state, setting the tone of practice.
Chapter 3 examines the idea of the holy, the saint. Reminescent of paganism, the holy man interceeded for all regardless of religion or creed. Such a man allowed the newly chrsitened, or reluctanly conjoled, to make a familiar trainsition into the new religion.