This book, Augustine of Hippo, is one of the better biographies of the great church father (Peter Brown’s new edition w/epilogue [Los Angeles: University of California Press, 2000]). This is a longer book (just over 500 pages), but it is well worth reading for an extensive and readable account of St. Augustine.
Here’s a summary of Brown’s section on Pelagius and Augustine.
“The basic difference between the two men…is to be found in two radically different views on the relation between man and God. It is summed up succinctly in their choice of language. Augustine had been fascinated by babies: the extent of their helplessness had grown upon him ever since he wrote the Confessions; and in the Confessions, he had no hesitation in likening his relation to God to that of a baby to its mother’s breast, utterly dependent, intimately involved in all the good and evil that might come from this, the only source of life.”
“The Pelagian, by contrast, was contemptuous of babies. ‘There is no more pressing admonition than this, that we should be called sons of God.’ To be a ‘son’ was to become an entirely separate person, no longer dependent upon one’s father, but capable of following out by one’s own power, the good deeds that he had commanded. The Pelagian was emancipatus a deo; it is a brilliant image taken from the language of Roman family law: freed from the all-embracing and claustrophobic rights of the father of a great family over his children, these sons had ‘come of age.’ They had been ‘released,’ as in Roman Law, from dependence upon the pater familias and could at last go out into the world as mature, free individuals, able to uphold in heroic deeds the good name of their illustrious ancestry: ‘Be ye perfect, even as Your Father in Heaven is perfect’” (p. 352-3).