What is to be remarked of such souls as have departed with faith, but without having had time to bring forth fruits worthy of repentance?
This: that they may be aided towards the attainment of a blessed resurrection by prayers offered in their behalf, especially such as are offered in union with the oblation of the bloodless sacrifice of the Body and Blood of Christ, and by works of mercy done in faith for their memory.
On what is this doctrine grounded?
On the constant tradition of the Catholic Church; the sources of which may be seen even in the Church of the Old Testament. Judas Maccabæus offered sacrifice for his men that had fallen. (2 Macc. 12: 43) Prayer for the departed has ever formed a fixed part of the divine Liturgy, from the first Liturgy of the Apostle James. St. Cyril of Jerusalem says: Very great will be the benefit to those souls for which prayer is offered at the moment when the holy and tremendous Sacrifice is lying in view. (Lect. Myst. v. 9.)
St. Basil the Great, in his prayers for Pentecost, says that the Lord vouchsafes to receive from us propitiatory prayers and sacrifices for those that are kept in Hades, and allows us the hope of obtaining for them peace, relief, and freedom. (The Longer Catechism 376-377)