30 We will pass over the other citations and rest content with recalling the testimony offered by St. Cyril of Jerusalem, who wrote the following memorable words for the neophytes whom he was instructing in the Christian faith: "After the spiritual sacrifice, the un-bloody act of worship, has been completed, we bend over this propitiatory offering and beg God to grant peace to all the Churches, to give harmony to the whole world, to bless our rulers, our soldiers and our companions, to aid the sick and afflicted, and in general to assist all those who stand in need; we all pray for all these intentions and we offer this victim for them . . . and last of all for our deceased holy forefathers and bishops and for all those who have lived among us. For we have a deep conviction that great help will be afforded those souls for whom prayers are offered while this holy and awesome victim is present." In support of this, this holy Doctor offers the example of a crown made for an emperor in order to win a pardon for some exiles, and he concludes his talk with these words: "In the same fashion, when we offer our prayers to God for the dead, even those who are sinners, we are not just making a crown but instead are offering Christ who was slaughtered for our sins, and thus begging the merciful God to take pity both on them and on ourselves.'' (21) St. Augustine attests that this custom of offering the "sacrifice which ransomed us" also for the dead was observed in the Church at Rome, (22) and he mentions at the same time that the universal Church observed this custom as something handed down from the Fathers. (23)
Source: Mysterium Fidei (Vatican.va)