Ambrosiaster is the name given to the writer of a commentary on St Paul's epistles, "brief in words but weighty in matter," and valuable for the criticism of the Latin text of the New Testament. The commentary itself was written during the papacy of Pope Damasus I, that is, between 366 and 384, and is considered an important document of the Latin text of Paul before the Vulgate of Jerome, and of the interpretation of Paul prior to Augustine of Hippo.
This commentary was erroneously attributed for a long time to St Ambrose. In 1527, though, Erasmus threw doubt on the accuracy of ascribing the authorship of this document to Ambrose. Erasmus is generally falsely credited for having coined the name "Ambrosiaster" (literally in Latin: "would-be Ambrose") to describe its author. The credit for this nickname should go to the Maurists, as René Hoven has shown. The name has remained with the unknown author. Attempts to identify this Ambrosiaster with known authors has continued, but with no success. Because Augustine cites Ambrosiaster's commentary on Romans 5.12 under the name of "Hilary", many critics have attempted to identify Ambroasiaster with one of the many writers named "Hilary" active in the period. In 1899, Germain Morin suggested that the writer was Isaac, a converted Jew and writer of a tract on the Trinity and Incarnation, who was exiled to Spain in 378-380 and then relapsed to Judaism; but he afterwards abandoned this theory of the authorship in favour of Decimus Hilarianus Hilarius, proconsul of Africa in 377. Alternatively, Paolo Angelo Ballerini attempted to sustain the traditional attribution of the work to Ambrose, in his complete edition of that Father's work. This is extremely problematic, though, since it would require Ambrose to have written the book before he became a bishop, and then added to it in later years, incorporating later remarks of Hilary of Poitiers on Romans. No identifications, therefore, have acquired lasting popularity with scholars, and his identity remains a mystery.
Several other minor works have been attributed to this same author, along with a lengthy collection of exegetical and polemical tractates, the Quaestiones Veteris et Novi Testamenti, which manuscripts have traditionally ascribed to Augustine. In 1905, Alexander Souter established that this work should also be attributed to Ambrosiaster. Fragments of several other works have been ascribed with some certainty to Ambrosiaster: a commentary on Matthew 24, a discussion of the parable of the three measures of flour into which a woman placed yeast, and a treatment of Peter's denial and the arrest of Jesus in Gethsemane. The attribution of other fragments to Ambrosiaster, though, is more tentative.
Despite the elusive identity of Ambrosiaster, several facts about him can be established. Internal evidence suggests he was active at Rome during the reign of Pope Damasus (366–384), and almost certainly a member of the Roman clergy. There are strong indications he objected to Jerome's efforts to revise the Old Latin versions of the Gospels, and that he was critical of Jerome's activity among ascetic women at Rome. Ambrosiaster shows a deep interest in Judaism and often notes that Christian practices derive from Jewish tradition.
Many scholars argue that Ambrosiaster's works were essentially Pelagian, although this is disputed. Pelagius cited him extensively. For example, Alfred Smith argued that Pelagius' "view of Predestination he seems to have taken from Ambrosiaster. His doctrine with regard to Original Sin appears to have come from the same source". However, Augustine also made use of Ambrosiaster's commentaries.