Erasmus in 1527 threw doubt on the accuracy of this ascription, and the author is usually spoken of as Ambrosiaster or pseudo-Ambrose. Owing to the fact that Augustine cites part of the commentary on Romans as by "Sanctus Hilarius" it has been ascribed by various critics at different times to almost every known Hilary. Dom G. Morin (Rev. d'hist. et de litt. religieuses, tom. iv. 97 f.) broke new ground by suggesting in 1899 that the writer was Isaac, a converted Jew, 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.
With this attribution Professor Alex. Souter, in his Study of Ambrosiaster (Cambridge Univ. Press, 1905), agrees. There is scarcely anything to be said for the possibility of Ambrose having written the book before he became a bishop, and added to it in later years, incorporating remarks of Hilary of Poitiers on Romans. The best presentation of the case for Ambrose is by P. A. Ballerini in his complete edition of that father's works.
In the book cited above Professor Souter also discusses the authorship of the Quaestiones Veteris et Novi Testamenti, which the manuscripts ascribe to Augustine. He concludes, on very thorough philological and other grounds, that this is with one possible slight exception the work of the same "Ambrosiaster." The same conclusion had been arrived at previously by Dom Morin.
Initial text from 1911 encyclopedia