From Dom Guéranger's The Liturgical Year.
Mark, successor to Sylvester the Pontiff of peace, has been honoured on this day from time immemorial. According to the testimony of St. Damasus, his virtues no less than his name recalled St. Mark the Evangelist.[1] He occupied the supreme See only eight months, but in that short time, he followed up the recent triumph of the Church by wise organizations. He built two new sanctuaries in Rome. He gave the pallium, of which this is the first mention in history, to the bishop of Ostia, to enhance his high privilege of being the appointed consecrator of the Roman Pontiffs.
This pontificate witnessed the awful death of Arius. Constantine had been deceived into ordering the reinstatement of this wicked man, who taught that the Word Incarnate was a mere creature. The heresiarch, followed by his partisans, was proceeding in triumph through the streets of Constantinople, intending to force open the doors of the basilica, where the faithful, with their bishop St. Alexander, were beseeching God, with fasting and tears, to avert the profanation. Suddenly, seized with an ignominious trembling, Arius was obliged to retire to a secret place, where his flatterers soon afterwards found him stretched upon the floor with his bowels cast out. He had merited the death of a Judas, for having delivered up the Son of God to the disputes of the people, to the mockeries of the proud, to the contradictions of the pretorium.
Prayer
Exaudi, Domine, preces nostras, et interveniente beato Marco, confessore tuo atque pontifice, indulgentiam nobis tribue placatus et pacem. Per Dominum.
Hear, O Lord, our prayers; and appeased by the intercession of blessed Mark, thy confessor and bishop, grant us pardon and peace. Through our Lord.