Tradition states that Antoninus, fleeing the Lombard invasions, headed for Campania where he ended up at Castellammare di Stabia.

The desire to remain a hermit himself led Antoninus to convince Catellus to return to his see.

Subsequently, Catellus was accused of witchcraft by a priest named Tibeius (Tibeio) of Stabia and was held captive at Rome until a new pope released him.

Antoninus became an abbot of the Benedictine monastery of San Agrippino, succeeding Boniface (Bonifacio) in this capacity.

Details of his life are based on an account written towards the end of the ninth century by an anonymous chronicler.