Among the correspondence of Augustin we find two letters addressed to that prelate by a certain Hilarius, of whom we know nothing certain except that he was a layman, an intimate friend of Prosper Aquitanus, an ardent admirer of the bishop of Hippo, and probably the person to whom the latter addressed his treatise, De Praedestinatione Sanctorum et de Dono Perseverantiae.
The first of these letters, which is short, is entitled De Pelagianis,
was written at Syracuse in A. D. 413 or 414, and is numbered clvi. in the collected epistles of Augustin, according to the Benedictine arrangement.
The second letter is considerably longer, is entitled De Semipelagianis,
was despatched from the south of France, along with one by Prosper upon the same subject, in 428 or 429, and is numbered ccxxvi.
It was published at Cologne in 1503, along with the treatise of Honorius Augustodunensis, De libero Arbitrio,
and is included in the Paris edition (1711) of the works of Prosper, p. 7.
Other possible works
A third letter was written by this same personage upon the same topics, which is now lost; and some critics have, upon no sufficient grounds, ascribed to him a work, De Vocatione Gentium.