Ambrosiaster a Pseudo-Ambrosius, the usual name of the unknown author of the Commentaria in 12 Epistolas B. Pauli, which is contained in the second volume of the Benedictine edition of the works of Ambrose. It appears from the book itself that it was compiled while Damasus was bishop of Rome. Augustine quotes a passage from this book, but ascribes it to St. Hilary, from which circumstance many have concluded that Hilary, a deacon of the Roman Church under Damasus, who joined the schism caused by Bishop Lucifer of Cagliari, was the author. But against this opinion it may be adduced that Augustine would not have given to a follower of Lucifer the title of saint. — Herzog, 1:277.