The Holy Ghost, who filled their hearts with the holy love of God and the hope of an eternal reward, sustained them in their torments (Epistle) and protected them in their tribulations (Introit). "Never," they exclaimed, "have we enjoyed such delights as those we feel in suffering for Jesus Christ."
After a day and night of suffering, they were pierced with arrows, in A.D. 286, and their souls "delivered like the sparrow from the bird-catcher's net" (Offertory) entered for ever " into the kingdom which had been prepared for them from the beginning of the world " (Communion).
On this day, the anniversary of the heavenly birth of these two holy martyrs, let us ask God, through their intercession, to deliver us from all the ills that threaten us (Collect).
Salus autem justorum a Domino: et protector eorum est in tempore tribulationis. * Noli aemulari in malignantibus: neque zelaveris facientes iniquitatem.
But the salvation of the just if from the Lord: and He is their protector in the time of trouble. * Be not emulous of evildoers; nor envy them that work iniquity.
(Psalm 36:39,1 from the Introit of Mass)
Praesta, quaesumus, omnipotens Deus, ut, qui sanctorum Martyrum tuorum Marci et Marcelliani natalitia colimus; a cunctis malis imminentibus eorum intercessionibus liberemur.
Grant, we beseech Thee, almighty God, that we who celebrate the heavenly birthday of Thy holy martyrs Mark and Marcellianus, may through their intercession, be delivered from all threatening evils.
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/09682a.htm