29 Moreover, the Apostle Paul, who faithfully transmitted to us what he had received from the Lord, (17) is clearly speaking of the Eucharistic Sacrifice when he points out that Christians ought not take part in pagan sacrifices, precisely because they have been made partakers of the table of the Lord. "Is not this cup we bless," he says, "a participation in Christ's Blood? Is not the Bread we break a participation in Christ's Body? . . . To drink the Lord's cup, and yet to drink the cup of evil spirits, to share the Lord's feast, and to share the feast of evil spirits, is impossible for you." (18) Foreshadowed by Malachias, (19) this new oblation of the New Testament has always been offered by the Church, in accordance with the teaching of Our Lord and the Apostles, "not only to atone for the sins and punishments and satisfactions of the living faithful and to appeal for their other needs, but also to help those who have died in Christ but have not yet been completely purified." (20)
Source: Mysterium Fidei (Vatican.va)