“To assist the souls in Purgatory,” says St. Francis de Sales, “is to perform the most excellent of the works of Mercy, or rather is is to practice in a most sublime manner all the works of Mercy together…”
This doctrine agrees very well with that of St. Thomas who says in his Summa, “Suffrages for the dead are more agreeable to God than suffrages for the living; because the former stand in more urgent need thereof, not being able to assist themselves, as are the living.” (Supplem., Q. 71, art. 5).
It was revealed to St. Bridget that he who delivers a soul from Purgatory has the same merit as if he delivered Jesus Christ Himself from captivity.
—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