Most Protestant churches have what they call an “open table” – which means that any baptized Christian is welcome to receive communion when they attend their services, even if they are not members of that community or denomination. These Christians are surprised when they attend Catholic Mass and are told they cannot receive Holy Communion.
The differences in practice are based on the differences in what we believe Holy Communion is and what Holy Communion does.
As Catholics we believe the Blessed Sacrament is Our Lord Himself, hidden under the signs of bread and wine. When anyone approaches to receive Holy Communion, the minister raises the Host and announces “The Body of Christ.” The communicant responds “Amen” to signify that he or she believes that this truly is the Body of Christ. For most Protestant Christians, communion is not understood to be the body and blood of Christ, but a sign or remembrance of His sacrifice on the cross. It would be dishonest for them to say “Amen” and to receive communion if they do not believe Christ is there. 
We also differ from Protestants in what we believe Holy Communion does. For most Protestants, the church is a purely spiritual reality to which all Christians belong by virtue of their faith in Jesus, regardless of denominational affiliation. The reception of communion is a sign of the unity of Christians in Christ. Since the unity signified does not depend upon denominational affinity, neither is this required for reception of communion.
For Catholics, however, the Church is more than a spiritual reality – it is a visible reality – with visible bonds of communion, including a common faith, common sacraments and united hierarchy. One becomes a member of the Church by receiving the Sacraments of Initiation – Baptism, Confirmation, and the Eucharist. Reception of Holy Communion is the culmination of membership in the Church – we become fully united to Christ’s Body, the Church, by being united to Christ’s Body in the Eucharist. To receive Holy Communion in the Catholic Church is to become a member of the Catholic Church. For this reason, those who do not wish to be members of the Catholic Church cannot receive communion in the Catholic Church.
It is not pleasant to remind our non-Catholic friends or relatives who are joining us for Mass that they cannot receive Holy Communion. But we should do so ahead of time, to avoid confusion or embarrassment at the time of Communion. We must invite them to pray with us that the obstacles to full, visible, unity among Christians may be overcome so that we may one day celebrate the Eucharist together.
 There are some limited circumstances in which non-Catholic Christians may be permitted to receive Holy Communion as described in Canon 844. One of the requirements is faith in the Real Presence.
 See Catechism of the Catholic Church 771, 815.
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