After the abdication of Diocletian in 305, and the accession in Rome of Maxentius to the throne of the Caesars in October of the following year, the Christians of the capital again enjoyed comparative peace.

Added to this were the dissensions within the church itself, caused by the large number of weaker members who had fallen away during the long period of active persecution and later, under the leadership of an apostate, violently demanded that they should be readmitted to communion without doing penance.

[2] According to the Liber Pontificalis, Marcellus divided the territorial administration of the church into twenty-five districts (tituli), appointing over each a priest, who saw to the preparation of the catechumens for baptism and directed the performance of public penances.

[2] The Liber Pontificalis says: "He established a cemetery on the Via Salaria, and he appointed 25 "title" churches as jurisdictions within the city of Rome to provide baptism and penance for the many who were converted among the pagans and burial for the martyrs.

Idem, "Roma sotterranea", II, 204–5) relates that Marcellus was looked upon as a wicked enemy by all the lapsed, because he insisted that they should perform the prescribed penance for their guilt.

This took place at the end of 308 or the beginning of 309 according to the passages cited above from the Catalogus Liberianus, which gives the length of the pontificate as no more than one year, six (or seven) months, and twenty days.

At the end of nine months he was set free by the clergy; but a matron named Lucina having had her house on the Via Lata consecrated by him as "titulus Marcelli" he was again condemned to the work of attending to the horses brought into the station, in which menial occupation he died.

Theodor Mommsen theorizes that Marcellus was not really a bishop, but a simple Roman presbyter to whom was committed the ecclesiastical administration during the latter part of the period of vacancy of the papal chair.