You might recognize these two from Eucharistic Prayer I. Pope Damasus heard the story of these martyrs from their executioner.
Marcellinus was a priest, Peter was not but was very good at bringing people to Christ, even his jailer. Marcellinus was good at baptizing the people that Peter brought to Christ. For that both were arrested, I would guess for disrupting the public service of public servants by making them Christian and therefore citizens unwilling to perform their public duties of offering sacrifices.
Whatever the case, they apparently even convinced the man who killed them and so much so that he would relate it to a young boy who would one day be pope for his inspiration.
Salvation is a cooperation of all of us, laity, religious, and clergy alike. Do not be afraid to aid in RCIA and be Peter to your pastor's Marcellinus!
We beseech you, O Lord, pour your grace into our hearts, that as we have known the Incarnation of your Son Jesus Christ by the message of an angel, so by His Cross and Passion we may be brought unto the glory of His Resurrection; through the same Christ our Lord. Amen.
Lord, so great is our love for you that even though we walk in a world where speaking your name can mean certain death your faithful still speak it and speak it all the louder.
Help us work for a world where all may speak their creeds and pray their prayers without fear of violence.
Hear the prayers of those who abide with you in dangerous times and in dark valleys, and who die with your name on their lips. Draw them quickly to your side where they might know eternal peace. Amen.
-- From the Angelus, and the Prayer for Christian Martyrs