Marcellinus was a priest and Peter was an exorcist, that is, someone authorized by the Church to deal with cases of demonic possession. They were beheaded during the persecution of Diocletian. Pope Damasus wrote an epitaph apparently based on the report of their executioner, and Constantine erected a basilica over the crypt in which they were buried in Rome. Numerous legends sprang from an early account of their death.
Sts. Marcellinus and Peter (d. 304)
Marcellinus and Peter were prominent enough in the memory of Church to be included among the saints of the Roman Canon. Mention of their names is optional in our present Eucharistic Prayer I.