It was very likely after 347, since he is not listed among the Gallic bishops who were present at the Council of Serdica in that year, a fact noted by Tillemont.

Commenting on this Liber contra Arianos, Butler remarks that it is "written in so masterly a manner, with such solidity, justness, and close reasoning, as to make us regret the loss of his other works.

"[4] Phoebadius figured prominently at the Council of Rimini in 359, where, along with Servatius of Tongeren, he advocated for the Nicaean position against the Arians.

That is the year when Jerome wrote his work De viris illustribus, which mentions that Phoebadius was alive, although of an advanced age.

The short biography provided by Jerome reads as follows: Phoebadius, Agenni Galliarum episcopus, edidit contra Arianos librum.