He combated the Eutychian heresy, ended the practice of consecrating bishops only in December, and sought to offset the effects of Germanic invasions.
When the Eutychians rose up in Antioch and installed Petrus Mongus, Simplicius made repeated complaints for action to Basiliscus and Leo I, emperors of the Eastern Roman Empire, for the restoration of the Catholic bishop; he did the same when Petrus Fullo usurped the seat of the patriarch of Alexandria.
[3] In 478, Simplicius held a synod in Rome, which pronounced anathemas against eastern heretical bishops Peter Fullo, John of Apamea, and Paul of Ephesus.
He saw the Heruliian mercenaries revolt, depose Romulus Augustulus, the last emperor of the Western Roman Empire, and proclaim Odoacer king of Italy in 476.
[9] Odoacer made few changes in the administration in Rome, leaving the city firmly in the hands of its bishop, Simplicius.