Saint Theonistus is a Catholic saint, venerated along with two of his companions, Tabra and Tabratha.

According to the 11th-century account, Theonistus, along with Alban of Mainz, Tabra, Tabratha and Ursus, attended a council in Carthage, and then went on a pilgrimage to Rome. They met Saint Ambrose at Milan and were sent to serve as missionaries to Gallia. Ursus was killed either at Aosta or Augsburg. Albinus was beheaded by the Arians at Mainz and was a cephalophore.

A miracle allowed Theonistus, Tabra, and Tabratha to escape from Mainz, and they managed to reach either Gothia or Gallia, and then reached Otranto or Sicily. Finally, they were martyred at Roncade or Altino by beheading, and were also said to have been cephalophores.

As evidenced by their African names, Tabra and Tabratha may have been African martyrs whose relics arrived at Altino or Treviso during the persecutions of the Arian Vandals. Theonistus' cultus in Italy is attested by the foundation of a monastery dedicated to him in 710.

The relics of Theonistus and his two companions may also have been enshrined with those associated with Liberalis of Treviso at the cathedral of Torcello after 639 AD.

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4. Saint Herbert
5. Saint Maximus