Pope Marcellinus (296-304) died a martyr and is one of many early Popes who are Catholic Saints. That fact alone leads me to the conclusion that Pope Marcellinus did not commit apostasy by sacrificing to the pagan gods, during the Roman persecution under Diocletian. The persecution began in 303, and he died a martyr in 304.
The more likely explanation is that the emperor and his supporters lied. They told the Christians that their leader, the Pope, had sacrificed to the gods, handed over the sacred books, and abandoned faith in Christ. They wished to deceive Christians into following the emperor’s decree. His goal was to extinguish Christianity, since the Roman pagan religion made emperors, like himself, into gods to be worshiped.
You see today how sinful secular society makes all manner of false accusations against Jesus himself, the Church, and the recent Popes. Do you think that an emperor who worshiped false gods, who persecuted and massacred Christians, and who hated the Church, would tell the truth about the Pope of that time?
Vatican I infallibly teaches that every successor of Peter, certainly including this Pope-Saint from the early Church, has the gift of a never-failing faith. And he is a Saint-Martyr. Therefore, we must conclude that these accusations against Pope Marcellinus are false, and that the truth is that he maintained the Christian faith unto death.