Modestus was an early christian writer mentioned briefly by Eusebius in his

Church History (Eusebius)

The Church History of Eusebius, the bishop of Caesarea was a 4th-century pioneer work giving a chronological account of the development of Early Christianity from the 1st century to the 4th century. It was written in Koine Greek, and survives also in Latin, Syriac and Armenian manuscripts...

 as the author of a book against the Marcion. Eusebius praises him as having "exposed the error of the man more clearly than the rest to the view of all".

Jerome

Jerome

Saint Jerome was a Roman Christian priest, confessor, theologian and historian, and who became a Doctor of the Church. He was the son of Eusebius, of the city of Stridon, which was on the border of Dalmatia and Pannonia...

 also wrote about him in De Viris Illustribus

De Viris Illustribus (Jerome)

De viris illustribus is a collection of short biographies of 135 authors, written in Latin, by the 4th century Latin Church Father Jerome. He completed this work at Bethlehem in 392-3 CE. The work consists of a prologue plus 135 chapters, each consisting of a brief biography. Jerome himself is...

(c. 32). He also states that a number of other works were extant in his time as being written by Modestus, but they were considered to be spurious.