“One of the holiest works, one of the best exercises of piety that we can practice in this world,” says St. Augustine, “is to offer sacrifices, alms, and prayers for the dead.” (Homil. 16). “The relief which we procure for the departed,” says St. Jerome, “obtains for us a like mercy.”
Considered in itself, prayer for the dead is a work of Faith, Charity, and frequently even of Justice.
—from Purgatory: Explained by the Lives and Legends of the Saints by Fr. F.X. Schouppe, S.J., Part Second: Purgatory, The Mystery of God’s Mercy – Ch. 34