From Living the Catechism of the Catholic Church: A Brief Commentary on the Catechism for Every Week of the Year: The Sacraments (Volume 2) (Ignatius Press, 2000), by Christoph Cardinal Schönborn:
"Baptism constitutes the foundation of communion among all Christians" (CCC, n. 1271). Anyone who is not baptized cannot share in the Lord's Supper. No one is excluded from the love of Christians, but only someone who has received the "washing of regeneration" can share in fellowship with them at the table of the Lord. But baptism is only the beginning, for it "seeks for the attainment of the fullness of life in Christ" (Directory for the Application of Principles and Norms on Ecumenism, n. 92), and thus for the common reception of Christ in the Eucharist.
Of course, separated Christians hold some sharply differing views about the meaning and importance of the Lord's Supper. These differences concern first and foremost the meaning of the priesthood, the Sacrifice of the Mass, and the Real Presence of Christ in the Eucharist (CCC, n. 1400). For Holy Communion to be true and fruitful, it must not be isolated from the totality of the eucharistic celebration.
There is now a quite simple and illuminating criterion for deciding whether a common reception of Holy Communion corresponds to the truth. When someone receives the Eucharist, he hears the words "The Body of Christ" and gives the response "Amen," "Yes, it is, I firmly believe it be so!" This Amen is preceded by the communal "Amen" at the end of the Eucharistic Prayer, after the words "Through Him, with Him, in Him." If the "Amen" to "The Body of Christ" is to ring true, then it must be in harmony with the "Amen" to the Eucharistic Prayer, through which the eucharistic Body of Christ has become present.
The "Amen" signifies assent to the offering of the sacrifice "in union with our Pope and our bishop," to fellowship with Mary and all the saints, "on whose intercession we rely," to prayer for "our departed brothers and sisters," and, above all, to the fact that bread and wine, by the power of the Holy Spirit and the words of Christ, spoken by the priest, "become for us the Body and Blood of [God's] Son, our Lord Jesus Christ."
Anyone who can give his Yes and Amen to these things is affirming the Eucharist as it is understood by the Catholic Church. He is saying Yes to the communion of this Catholic Church. His Yes and Amen to the fruit of this Eucharistic Prayer, the eucharistic Body of Christ, will also then be true. There are special situations in which Christians not in full communion with the Catholic Church can receive the Eucharist (CCC, n. 1401). The prerequisite for this will always be that they can say the twofold "Amen" with an upright heart.
Paragraph 1401 of the Catechism of the Catholic Church states:
Books by Cardinal Schönborn published in English by Ignatius Press:
• Chance or Purpose? Creation, Evolution, and a Rational Faith
• Behold, God's Son! Enountering Christ in the Gospel of Mark
• Introduction to the Catechism of the Catholic Church (co-author with Cardinal Ratzinger)
• God's Human Face: The Christ Icon
• From Death to Life: The Christian Journey
• Living the Catechism of the Catholic Church, vol 1
• Living the Catechism of the Catholic Church, vol 2
• Living the Catechism of the Catholic Church, vol 3
• Living the Catechism of the Catholic Church, vol 4
• Loving the Church: Retreat to John Paul II and the Papal Household
• My Jesus: Encountering Christ In The Gospel
Excerpts from books by Cardinal Schönborn:
• Jesus in the Gospel of Luke | The introduction to Jesus, The Divine Physician: Encountering Christ in the Gospel of Luke
• Excerpts from Chance or Purpose? | From Chance or Purpose?
• The Church Is the Goal of All Things | From Loving the Church
• Encountering Christ in the Gospel | From My Jesus
• Reincarnation: The Answer of Faith | From Death to Life: The Christian Journey
• A Shepherd Like No Other | From Behold, God's Son!