In the time of St. Bernard, a monk of Clairvaux appeared after his death to his brethren in religion to thank them for having delivered him from Purgatory. On being asked what had contributed most to free him from his torments, he led the inquirer to the church, where a priest was saying Mass. "Look," said he, "this is the means by which my deliverance has been effected; this is the power of God's mercy; this is the salutary sacrifice which takes away the sins of the world." Indeed, so great is the efficacy of this sacrifice to obtain relief for the souls in Purgatory that the application of all the good works which have been performed from the beginning of the world would not afford so much assistance to one of these souls as would be imparted by a single Mass.

The Blessed Eucharist: Our Greatest Treasure
Fr. Michael Muller, C.S.S.R.