Marcellinus and Peter were prominent enough in the memory of the Church to be included among the saints of the Roman Canon. Mention of their names is optional in our present Eucharistic Prayer I.
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 Emperor 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.
Why are these men included in our Eucharistic prayer, and given their own feast day, in spite of the fact that almost nothing is known about them? Probably because the Church respects its collective memory. They once sent an impulse of encouragement through the whole Church. They made the ultimate step of faith.
-Fransiscan Media