The offering of the Son to the Father for the sake of the souls in purgatory
Forasmuch as, in this divine sacrifice which is celebrated in the Mass, that same Christ is contained and immolated in an unbloody manner, who once offered Himself in a bloody manner on the altar of the cross; the holy Synod teaches, that this sacrifice is truly propritiatory and that by means thereof this is effected, that we obtain mercy, and find grace in seasonable aid, if we draw nigh unto God, contrite and penitent, with a sincere heart and upright faith, with fear and reverence. ___Trent
