See Orosius, 'De Arbit. Lib.,' and other authorities in Haddan and Stubbs.
Sidonius, Ep. ix. 3.
Constantius, the biographer of Germanus, says they were sent by a Council of Gallican Bishops; but Prosper of Aquitaine (who was in Rome at the time) declares they were commissioned by Pope Celestine. Both statements are probably true.